I don’t believe there is a FIRST endorsed site that will sell you more of these bearings. However a wonderful industrial supplier named mcmaster does.
Just head on over to http://www.mcmaster.com and do a search for ball bearings. They’ll have what your looking for. The only issue is that they have many, many different options. In general the cheapest bearing will probably be good enough for FIRST robots but I’d also suggest going over the options with a mentor. I’ve ordered bearings a number of times from them and i’m still not sure what is ideal.