Sheffield's Seed Company
The only source I was able to find for Great Basin bristlecone pine seeds.
Cole Hardware is an amazing place: a local store, crammed to the gills, so they'll have it when you need it. Four locations: 956 Cole, 3312 Mission, 70 4th St, and 2254 Polk. Recommended!
Link: Cole Hardware.
F. W. Schumacher
This site is the total bomb, if you are into tree seeds. After years of grubbing for sugar pine seeds, I visited this site and found that I could order them by the pound. Very highly recommended. Excellent reputation.
Link: F. W. Schumacher.
Maple Leaf Nursery
The owner of this nursery sometimes collects conifer seeds on his hiking trips, and is willing to part with them for a reasonable price. I value these seeds very highly because I know their precise origin. For example, I know that my Red Fir and Mountain Hemlock seedlings come from Carson Pass.