Measure the bearings in mm = Inside diameter X outside diameter x Width
(metric is whole numbers so it you measure 9.8something it is a 10)
Using the inside and outside diameters, Google the size such as "Bearing 10x30"
That will return you the trade number (such as 6200 for 10x30x9) that you can use with anyone who sells bearings (and if they go 'huh' you are shopping at the wrong place) the only thing is you need to know the quality. C3 is usual for what you need if they ask quality. Sealed or not? Open bearing in this case is just "6200" (inside a gearbox or crankcase), for sealed self lubed bearings you can get rubber sealed bearings "6200-2RS" or steel shielded "6200-2Z" (sometimes listed as 6200-zz).
If you are trying to source a bearing for a special project or want to retro loose balls, you can go to https://www.vxb.com/ and search around their site for all bearings for 11mm shaft, or 30mm OD, etc. The standardized trade numbers of the bearings will be same same between suppliers.