You can get them at Sears, Harbour Freight, pretty much any hardware store. Most of the threads are fine (UNF or the American equivalent TLA) i.e. #10-32, 1/4-28, 5/16-24, 3/8-24. There are a few fasteners bigger than that (rear shock mounts are 7/16-20, for example) and a few coarse threads (air cleaner to carbs are 5/16-18) but the above list covers pretty much everything. If you have a late TR6 then there are metric threads at the caliper, but probably not worth buying a set of metric taps for that!
Hope that helps
Alistair