Mechanical Keyboards

For you the row will only really apply to the right hand side since the "a" key will always be on row 3 and every key cap set will have "a" at the same height. The delete/home/end or whatever you decided to put on the right hand side above the arrow key will might be mismatched slightly but overall I don't think it's a big issue. On my keyboard my "r4" key is actually a "r3" key but it's close enough and it's only 1 key that's off.

KBDFans is probably the biggest store, but they do a lot of group buy stuff and sell extras after the group buy. You could do amazon, the massdrop/drop sets are a good starting place for keycaps. Otherwise most keycaps stores are specific to your country so for me in Canada I've got deskhero, ashkeebs and a couple others but the US has Kono, Canonkeys a bunch of others. You can check your region here for a list of stores: https://www.alexotos.com/keyboard-vendor-list/