Mister Fred said:
Try visiting your local PBS station. They may have some old ones in there library. You could proably even get a copy of one for a small charge.
You could try it if you like, but I seriously doubt it. TV stations and networks do not keep tapes lying around that they aren't going to be airing. They get erased and reused. Sesame Workshop, on the other hand, apparently does have a *complete* library (or at least a very extensive one) of the shows they've done since 1969. But in either case, unless you know someone on the inside, good luck getting anything out of them.
However, there are numerous episodes which have been going around between collectors.