Simply ask for a nine-pin Null Modem cable or Null Serial cable.

Should be enough, assuming the people at the other end of the transaction are high enough up on the evolutionary ladder. Depends on the shop you're going into.

If it were my local Radio Shack, the mouth-breathers behind the counter would probably get that deer-in-headlights look and call their manager. That's assuming I can even get their attention, since they are ALWAYS busy signing up someone for cell-phone service (a process which for some reason takes 45 minutes, during which every other customer in the store stands around and waits for them to finish since they are the only clerk in the store).

"Radio Shack. You've got questions, we've got blank stares."