The " bearing boys " website have a fantastic range of all sizes of bearing & types & excellent prices . Most bearings will have the part number engraved , stamped or chemically etched on the outer rim side face , if you use that number to order you should get a good like for like replacement but there are some other things you'll need to know when ordering to ensure you get teh exact replacement .
Some bearings also have a series of tiny rings etched on the rim just after the part numbers they indicate the quality of the bearing . The more rings the higher specification for temperature range , loading & speed they are designed for .
Take note ( Photograph s ?) of which way bearing come off shafts / out of seats as some are directionally loaded but are not taper roller bearing .
Bearing also have three options
1.
Open low cost where you can see the balls inside the carrier frames .
2.
Steel shielded where a pressed steel shield is covering the balls & carrier
3.
Sealed for life bearings which are filled with very high spec grease & ball bearings & have a rubber coated washers pressed in the sides of the bearing over the balls & carriers .. these bearings are usually but not always the most expensive and accurate bearing .
Sometime folk will have changed a bearing with whatever was available & it fails prematurely ... if your bearing housings have grease nipples they will usually be open or steel shielded bearings .