I wouldn't get your hopes up. Finding a PDP-8 of any description is
going to be fairly difficult in this day and age, even a PDP-8/A. I'd
assume that the majority of those that were made were either scrapped,
or are now in private hands, where the owners would be unwilling to part
with them. There may be someone on this list willing to part with one,
but i'd honestly doubt it.
You'll have better luck finding a PDP-11, but even those are getting
more and more scarce.