I've ordered quite a bit from keygem (https://keygem.com/) and oblotzky (https://oblotzky.industries/), and a little from candykeys (apparently dead now). They tend to have mostly groupbuy sets, so full set availability can be rough after the runs.

https://www.gmk.net/shop/ / https://www.gmk.net/shop/en/keycaps/ has sets ready to go, while they're apparently pretty good quality, they do look a bit plain (imo) - but each to their own!

some more mainstream sets can be found from eg. maxgaming (see their country/region specific sites). At least over here (Finland) some smaller computer stuff stores sell keycap sets (glorious, ducky... etc), like jimms.fi - but I'm fairly certain those are awailable pretty much everywhere.