Please don't get them at Petco. The pigs at Petco and pet stores in general are often mis-sexed and sick. The females can be pregnant from the mis-sexing, and their mothers are bred back to back until death. The conditions are deplorable and veterinary care is limited, so things like upper respiratory infections, mites and ringworm are all common. It would make sense to "rescue" them from the pet store, except when you buy a pig at a pet store, you're telling them that there is interest, and some female guinea pig somewhere is having more babies to replace the ones you bought, who then have to go through the whole thing again, so it's a never-ending cycle that only ends when you take your business elsewhere.
If you post your general location, i.e. nearest city, we can help you find them at a shelter or rescue, and you can also look on Craigslist. Sometimes a family member is allergic, or a baby is on the way and the humans aren't prepared for pets and babies, so most of the piggies are just unlucky and much healthier than pet store piggies on the whole. You can choose from so many more piggies, with varying ages, many of whom have been around people, and therefore are more social and friendly than the piggies at Petco. You can even find babies from the pet store accidents, or decide that maybe you're not really excited about babies, and get older cuddlier pigs. I've adopted all my piggies from Craigslist, and it's been awesome.
Sorry @lissie about the double post...
EDIT: If there are pigs sold at the pet store then there are families who find allergies, or are having life event where they don't want the piggies anymore. Please let us know your location and we'll do what we can to help you find rescue piggies
