Dennis Kirk, some parts for 350's. Mostly parts for maintainence; tires, tune up kits, gaskets.
Tim Holtan's Honda CB/CL350 Tech Tips
E-bay, lots of things and constantly changing. Watch your syntax when searching (CB350 will not find CB 350).
Cycle-re-cycle, Carb rebuilding kits, cables, gaskets and much more.
Honda305.com, site devoted to the 305 twin, the 350's predicessor.
Vintage Japanese motorcycle club, all Japanese bikes 15 years old or older are included.
A great site devoted to the SL350, check out Barry's progress as he restores his own SL350K0.
Lot's of inexpensive 350 parts, some NOS but most are used.
An article on how to build a 350 racer
Retro Bikes - A nice selection of Honda NOS (New Old Stock) parts
Or you can search the web yourself.....