There are a lot of vendors that sell full keycap sets. Here is one: https://mechanicalkeyboards.com/
There are various types of keycaps. They can all be one size, they can be different heights on each row, they can have letters printed on ($) or they can be several layers of different colored plastic with the letters being part of the molding process ($$$).
Good luck! Happy to answer any other questions that might come up.