There are guinea pig "deserts" where there aren't any/many rescues, shelters, whatever. My suggestion is to use Petfinder to find the rescues closest to you, and contact them to tell them what you want. Rescues are almost always just a group of foster homes, and whoever does the bookkeeping has the address. But they might have a pig much closer to you than their address indicates. Also, they'll sometimes meet you halfway, or arrange for a piggie train (volunteers will drive the pig an hour or so in the right direction, then hand it off to the next person).
Also don't forget to check any city/county animal shelters in your general area. Look for other kinds of rescues (rabbits, ferrets, etc) -- people have been known to dump a different kind of animal than a shelter usually takes because they know anybody kind-hearted enough to run a shelter will take another kind of animal in.
Good luck with your search.