I don't have a lot of trusted online supply sources, but I'd like to start sharing some of the few that I have found. Buying supplies online, like many other things, can be difficult because often you can't truly tell the quality of the product until you're able to hold it and give it close inspection. Last Saturday, I posted a new necklace I made using a beautiful vintage focal bead I purchased from mksupplies [MK Beads] on Etsy. I was very impressed with both the bead and the service I received from this husband and wife team, so I did a bit of research after posting and discovered they recently opened their own webstore.
MK Beads offers a wide range of vintage and contemporary jewelry supplies. They have one of the most extensive vintage collections I've seen, with dozens of shapes, sizes, styles and colors. My favorite sections include their plastic and lucite beads, plastic cabochons, glass cabochons, and cameos. Their contemporary collection includes etched plastic beads, Czech glass beads in many shapes and colors, and a large selection of lovely plastic floral cabochons. You'll also find chain, charms, settings and other findings in both collections. The site itself is beautiful and very easy to navigate, and although I haven't ordered directly from it yet, I feel very comfortable doing so after my experience with MK Beads on Etsy. Bonus - right now they're offering 10% of your order!
If you use ArtFire, you can also find MK Beads there under mksupplies.