Many thanks to Paul G. for giving us a forum for Tree mills.
The two sources of Tree parts that I know of are:
Ellison Machinery (http://www.ellisonmachinery.com/), though I believe their midwest branch (http://www.ellisonmw.com/) is the one that handles Tree parts.
3 S Incorporated (http://www.3sincorporated.com/), which is run by a bunch of ex-Tree guys who purchased the machine shop, fixtures and equipment from Tree. They stock spare parts and do spindle rebuilds. They also have prints for a number of parts and can replicate spares from scratch if needed. Their prices seem a little better than Ellison as well - Ellison wants $230 for a spindle nose ring while 3S is only charging $175.