I'm from St. Louis. I know that there is St. Louis Animal Harbor
, and they regularly list guinea pigs on petfinder.com. The woman who runs the rescue, Suzie, was a member of Guinea Lynx, but I haven't seen her on here in a while. They don't have any listed right now, but keep checking on petfinder. The Humane Society in Maryland Heights and the one near downtown often have pigs too, which are also listed on petfinder.com.
I adopted my two girls from the Humane Society in Maryland Heights. Libbylou, who is a member here, volunteers there.