From 1980+ the valve shims were replaced with adjustable valves, and didn't require shims put in, so you'll have to talk to 79 and below guys to try to find some shims laying around. Also, if you have any somewhat local motorcycle junk yard for older bikes, im sure you could get them there for cheap or nothing.
Also, check out bikebandit.com and z1enterprises, im sure they're cheap to buy.