Almost all contemporary mechanical switches you see are Cherry MX-compatible, both when mounting to keycaps and mounting into PCB and plate. Many switches can also exchange internal parts.
The exceptions are the Omron (Romer-G), Matias' switches (clones of vintage Alps) and a couple low-profile switch types.
So compatibility shouldn't be a problem. The only issue is that some switches have plastic legs for extra stability and those won't fit into all PCBs: and the solution to that is easy: snap them off.
There are quite a few sites out there that cater to the DIY keyboard hobby, but few sell keyboards parts for TKL size or up.
KbdFans in China is the biggest keyboard parts shop, and they have only up to 75%.
There are however many keyboards made only temporarily as Group Buys on various forums. They often first start with an "Interest Check", to gauge interest before starting proper. Some of these are posted here, but there are more of them on Geekhack.
The most popular firmware right now is
QMK. With that you change the layout by flashing a new firmware to the keyboard.
Another approach: If you don't mind Drop (formerly "Massdrop"), you could instead get the
Massdrop CTRL keyboard. It is a new version of the
Input Club K-Type with another name, colour scheme and firmware (QMK instead of Input Club's
Kibohd firmware).
It has hot-swapping sockets for swapping Cherry MX-compatible key-switches, proper aluminium construction (not just sheet metal), RGB backlight, sideglow and programmable firmware. But you won't actually build anything...
The original Kibohd firmware allowed also reprogramming over a serial interface to the keyboard.
