Any local bearing house should have them. Or you can get them online at a place like McMaster Carr. You will have to know the industry part number, as described above (most likely a 608 or 698) or atleast the ID/OD/W.
If you get me any of the above, I can give you a part number to take to a bearing house (Applied Industrial/Motion Industries/Kaman etc...)
On my Rizoma's, they just used cheapo Chinese bearings, but I interchanged them and procurred some (I'm a bearing sales engineer, so it's easy for me).
I would also recommend getting something with contact seals. The shields are okay, but still allow water to get in there and rust the bearings. That is what killed the bearings in my Rizoma's, and that was with only riding in the rain and washing. It's stored inside.