I bought both 10 and 14 gauge THHN by the foot at Home Depot, but that was only after screwing up and wiring 2/3rds of my panel with low voltage primary wire. That was a fun night when I realized what I had done!
Mine is a simple 1 element, 1 PID, 1 pump eBIAB, so YMMV, but I used about 15...