I generally mail order specific plants. Hauserman's is a favorite and the prices can be good for the sale plants. I've also found that Carter & Holmes has bench clearing specials in the summer. I bought 12 plants for $90 with free shipping last summer. Of course, they are 2-3 years from blooming.
For spur of the moment purchases
, I like Trader Joe's, Whole Foods, and local nurserys with the post blooming table at 50% off, though I often find that these are no bargain even at 50% off. They key is to choose a healthy plant not just one with lots of blooms.
I'm heading to Southern California in November and plan to hit the Orchid Trail to see what I can find.