A reputable and respectable parts supplier will include details on how to differentiate between the various keyboard models. You just have to search until you find a vendor who provides that information. Typically there are just two styles of key caps/scissor mechanisms for each model year (there may be exceptions). You will want to know your exact model as well which you can get by clicking the Apple menu and selecting "About This Mac".
You will want to make sure the scissor mechanism and metal support brackets are not damaged plus you want to make sure the rubber "nipple" is present in the center of the key area.
I don't know who to recommend since I just scavenge used key caps from old keyboards that no longer work. Maybe you can ask at iFixIt.