what he said...and good luck. i've been looking for a couple of months...
one thing you might want to do before you start looking is try to learn all you can about the one you have (if it's original) or the one that is supposed to be on it... lots of the seat pans have different patterns on them, little groove or holes in particular places.
this can be important when you are looking at some of the horrible, blurry photos on Ebay...
- and dont forget that after 71 the pans were hinged on the left, not the right side as they were from 69 - 71 and had a different latch...at least on the 750's...
i haven't been able to find a solid pan source, seems hit or miss...