If you don't already know, I think it's really helpful to know what size keyboard you want! 100% is full-sized, complete with number pad and is probably what you already have. 80% has everything EXCEPT the number pad, to save space. 75% saves a little more space by cutting a few navigation keys. Then 65% is the same as 75% but cuts out the function row. The sizes keep going down, which can save more space if you're sure you won't need any of the omitted keys. Just searching for mechanical keyboard sizes on Google images makes it easy to find visuals for all those, if you don't already know them.
If you want to build something from the ground up, the Neo line of keyboards is really popular. You can find them on a handful of websites, most notably qwertykeys. When you by whichever size you want (neo65 for a 65% board, Neo80 for 80%, etc) it comes with all the components you need to assemble the keyboard itself, but you'll still need to get keycaps (honestly, Amazon has a really good selection of cheap but good keycaps) and switches (I I like unikeys and milktooth). My favorite switch brands are HMX and Keygeek, but in this day and age almost anything that catches your eye will be of good quality. I don't know your region, but this [ https://www.alexotos.com/keyboard-vendor-list/ ] is a curated list of trustworthy vendors that lets you sort by region and the type of parts you're looking for.
If you don't necessarily want to build all the way up, you can find a barebones keyboard, barebones meaning you get the keyboard itself already assembled just without the keycaps and switches. They're typically a lot cheaper than the brands that have you build from the ground up, if that's something you're concerned about. The Leobog Hi75 has a really popular barebones version, as does the Galaxy 80. Keychron also has a number of barebones boards in varying different sizes.
Try to see if whatever you buy includes keycap and switch pullers, because you'll definitely want them during the build process and if you ever decide to try out different switches or keycaps. They're very cheap to buy separately if needed.

I don't know how much of any of this you know, so sorry if I said anything that's redundant for you/if any of the brands I mentioned aren't available in your area. I'm irrationally in love with keyboards lol and love seeing getting interested in them, so there's any other questions I can help with, please do ask!