UNI has claret mohair on spools. I would think any fly supplier who sells UNI can help you. I also buy mohair in yarn shops, Joann's Fabrics etc. if that helps.
NOTE: Instead of Claret, UNI calls it Wine.
Caddisflyshop in Oregon has it.
There are a number of materials and techniques used to make leeches. Mohair is one of the best. Easy to tie, can be "picked out", and it moves and looks good when wet.
I'm sure there are other brands besides UNI but it is the only one I am familiar with.
Also used in atlantic salmon flies. Of course, more modern uses are detailed above, including the woolly bugger variant.
This one, while abysmally out of proportion (one of my early flies) has mohair wisps above the body sections. Many other salmon fly patterns use mohair dubbed for the body material.