RE: caller??????
I've flown in only a small handful of IMAC contests, and don't have a regular caller at all. Just go to the contest and talk to the guys there, you shouldn't have any real trouble finding someone to call. Even if it's a different person each round (which I've done).
I will say that your best bet for pick-up callers is usually someone in the same class you're flying, so they are already familiar with the sequence. Often, once you see the flight order for a round, you can ask a pilot who is flying a few spots before you or after you for help calling. (The guy right before you will be busy landing and shutting down, the guy right after you will be busy getting ready to fly most of the time).