Try Van Engelen, which is the wholesale side of John Scheepers.. Quality is excellent, and prices are reasonable.

You will find more availability of lilies in the fall, however, as that is when they are freshly dug. Bulbs for spring planting tend to be the tender, non-hardy type, like dahlias.

I like Hallson Gardens for lily bulbs (www.perennialnursery.com). Good pricing, nice bulbs, and excellent customer service as well!