You can buy them on Aliexpress for $5. YouTube and a cursory google search will give you many many options to choose from for examples and tutorials.

There are a lot of radar modules out there in the wild.

But not all of them are good for doing stuff like this.

You need full raw I/Q and DAC access to sweep the frequency.

don't get the $5 ones, they are probably just presence detector or at best distance detector and probably work on 24Ghz. Get the 60Ghz for breathing, heart rate, posture etc.