Not specifically PreGen experience, but you can also just try Googling the part number you're looking for. Check Amazon too, both direct and their third-party sellers. Personally, I've found most of the standard bike sites to be within a few bucks of each other on the same tire whenever I needed some. The best deal usually comes from whichever one happens to have a sale or coupon code at the time.
JakeWilson has ended up being the overall cheapest for me a couple times now, though it was only a few bucks better than the next one.