Hi Nitocris. Welcome to Geekhack.
There are a few places to look. A lot of people seem to like kbdfans. I have used KPrepublic for a few purchases myself. There's also mechanicalkeyboards.com, and aliexpress (and other Chinese websites). NovelKeys has some good switch prices. There are also group buys posted in their own section on this forum, and Drop. I'm sure I'm missing a ton.
I ordered a few
Heavy Metal Keyboards myself.
I don't do a lot of the kit/LEGO stuff, so others would know better than me. If you set specific parameters and listed them in the keyboards section of the forum to ask for recommendations, you'll probably get more views and responses.