My favorite sources for seed besides the NARGS seedex are www.rmrp.com "Rocky Mountain Rare Plants" and www.Alplains.com "Alplains". You'll have to call or e-mail Alplains for a catalogue as their website isn't up yet.
I've also been looking at the following as well... still haven't ordered from them but they have some very interesting offerings:
www.jjaseeds.com/ "Jim and Jenny Archibalds site"
www.gardensnorth.com/site/index.htm "Gardens North"
www.silverhillseeds.co.za/ "Silverhill Seed"
www.mesagarden.com "Mesa Gardens" Cacti and assort. succulents
Another favorite plants source is www.rockgardenplants.com/"Beaver Creek Greenhouse"
If you are a member of NARGS the quarterlies also have advertisments with information to several other sources that I have yet to try.