I sample myself, often recording long guitar or bass improvs, and then working through them to extract loopable slices. (I use Reaper for this.)
I take assorted samples and mix them up in Renoise, trying to get those serendipitous moments.
That’s pretty much how I assembled my Small Guitar Pieces album:https://jamesbritt.bandcamp.com/album/small-guitar-pieces
Of course, a lot times there are loops that feel like they should play well together but don’t because they’re not quite sitting well.
There’s a free utility called Sox that runs on Windows/MacOS/*nix.
I use it to repitch (and sometimes re-tempo) samples.
There’s some math involved in getting things Just So. : ) However, if you can write small scripts you can adjust a large number of samples quickly,
There was a discussion here recently about sample editing tools:https://forum.renoise.com/t/sample-editing-tool/49324
Something there might be of use for this as well.
But I just use my ear, trying samples at different pitches, to see what works. I like the surprises.