There are basically 3 kinds of multiplier onions:

- Root multipliers, which includes shallots, potato onions, and "nesting onions". For these, you plant one bulb, which then divides to form a cluster of bulbs. Many of these onion bulbs have good storage characteristics, and can be over wintered indoors for Spring planting... but will usually perform better if Fall planted. Some of these were the predecessors of our modern onions.

- Top-setting multipliers, a.k.a. "walking onions" (including Egyptian onions). These grow a "flower stalk" with clusters of small bulbs at the end, and usually form only small ground bulbs with very poor storage characteristics. Hardy perennials that are best used for scallions. They can be harvested in both early Spring and late Fall; they are the first & last vegetable from my garden each year.

The two above are sub-species of the common onion.

- Bunching onions, which are a different species (Allium fistulosum). These do not form bulbs, and are only used as scallions. Some varieties have very wide stalks (1" or more) and/or have been bred for long stalks. They are very popular in the Orient, which has spent more effort in breeding new varieties. These are clumping perennials which divide to varying degrees, with smaller varieties dividing rapidly, and larger varieties dividing slowly.

Root multipliers are generally sold as bulb stock. You could purchase & plant store-bought shallots; if you do so, inspect them carefully for signs of spoilage. Territorial Seed has a large selection of shallots, they sell bulbs for several varieties, as well as shallot seed for several others. Yellow potato onions are sold by several sources; you will usually find them listed along with garlic. I had hoped to list Southern Exposure Seed Exchange as a source, since I know they have carried potato onions in the past & are in your climate zone... but I've been unable to pull up their website.

Walking onions are generally started from the bulbils which form at the end of the stalks. These are available in late Summer or Fall. A few commercial sources offer them occasionally, but you are most likely to get them in trade from other gardeners. Treat them as perennials; I have a healthy clump which has been in the same location for 20 years. You can multiply them by planting the bulbils which are produced; or if you cut those off before they develop, the bulbs at the base will usually divide.

Bunching onions are usually started from seed, although you can also grow them from divisions of established clumps, transplanted in Spring. Evergreen Seeds has one of the better selections (no link, their site is temporarily closed until March 17th). Baker Creek also carries several varieties.

I grow an heirloom bunching onion which is a good compromise between large size (1/2" stalks) and rapid multiplication; if you contact me in June/July, I'd be happy to send you some seed.