Autozone, Checker/O'Reilly's, NAPA, etc.
Seriously, did you even try looking? Not trying to be insulting, I just find it hard to believe that you don't have a big-name auto parts chain out there that would carry something that common and simple.
Did you go to an Autozone and ask for an IDLER pulley for an '88 5.0? There's a TENSIONER pulley on there, but not an IDLER pulley. Most of the idiots they hire for those stores don't know anything other than what they can look up in their computer, so if he only looked for an idler pulley, of course he wouldn't have found anything.
If that's the case, go back there, try to get a different employee to look up the part for you, and ask for a tensioner pulley, instead. DO NOT let them try to tell you that you can only buy the tensioner and pulley together as a fully assembly - I've had that kind of idiocy pulled on me before at Autozone.
Oh yeah, and don't forget that the bolt on there is a reverse (left-hand) thread.