You will probably have to join a group buy for keycaps that you like, that is running right now or soon. Another option is to try to buy second hand, although that can be significantly more expensive depending on the set.
This keyboard uses a few non standard sized keys, mainly the 2.25 and 2.75 spacebar keys and the 1.75 ANSI enter. Most keycap sets today are offered with a base option which covers the most standard keyboard sizes and with optional sets you can purchase, for example a spacebars kits will often have these spacebars that you need, or there will be a 40s set for small factor keyboards. Make sure that the 1.75 unit ANSI enter is provided, or a similar key (maybe novelty) is being offered, because sometimes even these optional kits won't offer it (and sometimes it's offered in the base kit, you will really have to check the specific keycap set that interests you).
