You won't find them in stores for fall for differing reasons that relate to how they grow.

With the potatoes, because of the timing of the planting, growth and harvesting of the spring/summer crop, potatoes from that crop cannot be used for fall planting because they would have to undergo a lengthy dormancy of several months. The length of the dormancy is longer than the time frame between our spring/early summer harvesting and our fall potato planting time. Without that dormancy, they won't sprout. Thus, commercial growers cannot produce seed potatoes at the timing needed for fall planting in our zone. This also explains why, when you miss some small potatoes while digging, they generally don't sprout in the same year, but do sprout the next winter/early spring as the soil warms up. By then, their dormancy needs have been met.

You have a couple of options, only one of which is viable for this year.

1) Purchase regular grocery store potatoes and plant them. Your best chances of success with this method would involve purchasing organic potatoes. The reason why is that conventionally-grown potatoes usually are sprayed with a chemical that inhibits sprouting. Although it eventually wears off and the potatoes do eventually sprout, it can take a really long time for the anti-sprouting sprays to wear off. Organic potatoes are not sprayed with these chemicals so sprout more quickly. When I do this, I like to buy the potatoes 2 or 3 months before I intend to plant them and put them in the back, darkest corner of the walk-in pantry so they can sit there and eventually sprout, hopefully by the time I want to plant them.

2) Buy your fall seed potatoes in late spring while they're still in the stores. Usually I see seed potatoes in the stores through at least May, even though I bought and planted my seed potatoes in February or March. If you buy them in May, and store them in a cool, dry location, they should survive long enough for you to plant them for fall.

3) Order online from retailers who ship them. A few of the online retailers ship as late as June, although you'd need to order them earlier than that (so all the good ones aren't sold out).

Onions are a whole different story. Almost all the bulbing types we grow are daylength-sensitive, with the number of hours of sunlight in the day being the factor that triggers them to bulb up. We plant them while the daylength is relatively short in winter, and the lengthening day-length from May through July triggers them to bulb up. In the fall, our weather is all wrong for bulbing onions because, if you could find them, you'd plant them in the longer daylength of summer and they'd be exposed to decreasing daylength as the days went on. The most likely result, depending on when you planted them, would be that the summer daylength would trigger them to try to bulb up very early in their lives when they didn't have enough stored energy in their leaves to produce a bulb. Or, you'd get perpetual leaf growth that never would bulb up.

The biggest onion plant producing farm in the USA, Dixondale Farms, doesn't even try to ship onions for fall planting in our zone. I believe they usually start shipping in October or maybe early November, which is great for people in warmer zones than ours, but doesn't work for us.

For fall onions, you can plant bunching onions and use them as scallions. These can be easily grown from seed and you can harvest them anytime you want. They won't give you big bulbs, but they produce tons of small ones.

Fall also is the time to plant perennial onions, shallots and garlic.

There are some daylength-neutral onion varieties that do not rely on daylength to trigger bulbing, but you still won't find them sold in bundles of plants at this time of year like we see in the spring. You could raise them from seed, but I don't think they'd have enough time in summer/autumn to produce a big bulb like we get in spring/summer. Remember that in autumn, the daylength is constantly decreasing and so are the soil temperatures and air temperatures, so it takes fall crops longer to mature than in the spring and summer. The intensity of the sunlight also is lower, so that makes them grow even more slowly. However, you might get onions that were large enough to be worth the time you spent growing them.

Thanks for the knowledge. Tried potatoes and onions last fall; potatoes did well, but onions were just like you said, very small, some were nothing more than leaves. Thanks again!

Was looking forward to adding perennial leeks to my garden but got the email from Southern Exposure Seed Exchange that the crop failed due to flooding in TX so I'd get shallots instead. Don't get me wrong, I love shallots, but I'm disappointed none-the-less. I've actually only ever grown shallots in the spring so I'm excited to try a fall round.

hoon67, it may not be helpful this season, but Baker Creek Heirloom (www.rareseeds.com) might be another source to try. I don't think they have potatoes, but I know they have lots of onions. They only have seeds/bulbs, except for a short period of time in spring where they have a few live plants of some things available.

Kate, Baker Creek has leek seeds. I've never grown leeks, so I don't know if starting then now from seed would work, but they do have a few varieties. http://www.rareseeds.com/store/vegetables/leeks/

A tip: If politics are not your thing, or you just don't want to deal with GMO/gene altering/etc. politics, I'd advise either staying away from the Baker Creek FaceBook page or ignoring all comments/posts not made be Baker Creek. The company is very much anti-GMO and anti-Monsanto (which is fine with me personally, I buy their seeds because they work for me and are open pollinated and non-hybrids as well as all Heirloom varieties, and don't feel the need to deal with the politics and such on the page), which is very obvious on the FB page, and which also means that the FB page has quite a lot of people who post on it specifically to tout the benefits of GMOs and Monsanto whenever they can. And any questions should be either emailed or called in, because the FB page is run by what amounts to a company media specialist.

Kim