Yeah, the competency level of the average bike technician doesn't inspire my confidence either. If it's not written in their repair book WITH PICTURES they are lost!
I always save chainring spacers - you never know when you're gonna need an 1/8" adjustment. But it would be a total "trial and error" job on a triple crank with mixed brand rings. I've done stuff like this - but it's usually when I've got no other options. Better to find a FSA chainring on ebay or something.
Have you tried your local bicycle salvage? We have one in my hometown, and they have TONS of stuff you can't find anywhere else. And usually for dirt cheap. In fact, I'm going there tomorrow to scrounge up some old rattrap pedals!