Avidrc.com has the 12 x 16, and 6 x 13 x 5, 1 buck each. You can run standard 6 x 12 x 4's with the reducer sleeve, I have no idea why Traxxas did that, 6 x 13 is not a standard size. You can try Amain, Acer and vxb.com also if you need more options.
While you are waiting for your new bearings, try this- take a sharp X acto blade and gently get under the edge of the seals on your bad bearings and pry them up. Blast them out with a high solvent and turn them a few times until the debris is clear, I use oil instead of grease, anything between 30 and 90 weight should be fine. Once you do a few they are easy, and in a pinch you'll be able to run them when you can't wait for replacements.
I just replaced all of the bearing ins my E-revo with Ceramics and kept all the old ones, there yours if you pay postage (there is one missing axle bearing as it exploded, that was the excuse for me getting the Acer's!.
Toys:
Slash with VXL
E-revo with MMM/Teckno Nue 1515/1yf running 4s and 5s
Sweet, thanks Linc, now I have a link incase I can't Shizzon doesn't have the right bearings (He said one exploded, and if it's the larger one, then I'll need to shop around, but if the smaller bearing exploded, I can use the old bearings from the old knuckles, I'll just need the oversized (larger) bearings)