In our zones, fall and early winter are best - October through any time the soil is still workable seems to work for me in zone 4. Lily bulbs don't show up in stores here until early fall of each season - and if they do show up in the spring, they're likely last year's bulbs.

I don't mail order so can't help you with that. But many people on these forums do and should have some good sources to recommend. I've purchased Asiatic and Oriental lily bulbs from many places from Wal-mart to Home Depot to garden centers and they've all done well regardless of where they've come from. My best Asiatics were purchased as grown, about to bloom plants from Home Depot - they've proven to be good multipliers in just the few seasons I've had them.

The Lily Garden is one of my favorite mail order sources. A great selection and great bulbs.bbb

Since lilies are summer flowering bulbs, they can very easily be planted in spring for summer color. Many of the best online vendors ship in spring, especially to colder winter climates. These are not 'old' bulbs but freshly harvested during the winter months to right before shipping, as these are grown in mild winter climates that allow for that ability. I'd be a bit more cautious regarding packaged lily bulbs found in stores and discounters as they do tend to be harvested in fall. Although most of these are maintained under careful storage conditions until they appear in retailers for sale, not all of these suppliers are as careful as they should be. If the bulbs are firm and appear healthy, there is no reason NOT to purchase and plant in spring.

Here are a couple of the better online vendors I happen to be familiar with, as they are local :-)

B&D Lilies:
http://bdlilies.com/

The Lily Pad:
http://lilypadbulbs.com/