Hi, I buy mine onlline at M&J trimmings. They have a marvelous selection in both size and color and, if you are going to do a lot of them, buying them in bulk really saves A LOT of money. Here's the link:
http://www.mjtrim.com/
I have found the genuine Swarovski crystals work better (have a better glue) and seem to sparkle more than other brands, but I haven't tried all of them, of course.
Also, if you have a mirror with a frame that you can place flat on the table, you can put your crystals on the mirror and pick up your crystals on your iron and see when the bubbles start. Doesn't take long. Also, if you get a nice long pin to help push down through the slits of the tips if they get stuck and to help place them before they are permanently placed, you can get them in the right place better.
I buy other trims there too, just fyi.