If you're getting flasks/compots, Flasks by Chuck Acker is good. Limited selection, but really top of the line in terms of U.S. breeding and reasonable prices for most things.
Fairly recently I've purchased some things from Piping Rock. They haven't bloomed yet, but the roots and foliage were very healthy and the plants were of advertised size (i.e. blooming size). On a side note, I've been a little shocked by the number of retailers that list their plants as blooming size or near blooming size, but you receive plants that are anything but. So, I now use that as a test of quality, and Piping Rock delivered. Also, a lot of the prices are very affordable for a slipper, and I plan to purchase more plants, soon.
It's a little more hit and miss, but Ebay can also be a good source. Just make sure to do your research on the seller and be sure to review the ratings and feedback.
Also, one of the sub-forums on this site is for vendors, so you might want to check those out. They all seem pretty good.