Used parts:
http://www.baxtercycle.com/
Crashbars, big brake kits, other bits:
http://www.jacklilley.com/newcart/
Crashbars:
http://www.thunderbike.co.nz/index.htm
Misc bits:
http://shop.wemoto.com/index.dyn?oid=5834447
http://www.tm-accessories.de/shop/in...00--T400-.html
Things like chains/sprockets, handlebars, risers, heated grip kits, etc can pretty much be gotten anywhere. Several exhaust options - search here and triumph-tiger.com if you get a Steamer and decide to make it loud (same goes for windscreens). Corbin makes a seat you can buy right off the shelf.
I can't remember who makes them, but not so long ago there was some talk about hard bags and frames. I think maybe it was Hepco & Becker but I only see 955 and 1050 links now:
http://www.hepco-becker.de/_eng/frameset.php
Still some options out there for parts...but disappearing slowly, I'm sure...