As said above, we highly suggest rescuing a Guinea pig but with that in mind I can't exactly say YOU MUST RESCUE because both Mo and Steve aren't from a rescue- though they aren't from a petshop either.
However,I would advise against petsmart because I've heard of many people obtaining guinea pigs who are already sick and die only days after purchase.
Rescuing a guinea pig allows you to know the piggies health background,favourite food and personality. They are likely to be tamer and healthier and some come already bonded so need for the worry of introducing a new guinea pig. It also allows you to know that you've given a guinea pig a loving home who needed one.
Petshops stock young,baby guinea pigs because they are cute and SELL with it being a SHOP they want to make money and many don't know how the animal should be properly kept. Many get the baby guinea pigs from cheap breeders who usually mass breed and sell them on at a cheaper price. Thus many come sick to the petshop and therefore to their new home ill. However, some people get guinea pigs from petshops that are healthy and they get too see them grow etc and tame them themselves so I can't say every petshop is the same.
Breeders are much like petshops there's good and there's bad. Some breeders again do it only for money and don't care about the guinea pigs welfare. They breed purely for money and sometimes even interbreed with guinea pigs from the same family leading to faulty genes or sick babies. Some accidentally get babies from getting a misexed pair of guinea pigs or buying an already pregnant guinea pig and wish only that the babies go to good loving homes. In my case I trusted the breeder because I had had a sow from him before and though I did feel guilty going to a breeder as opposed to a rescue I did feel better knowing that I trusted the breeder and knew he wasn't in it purely for the money and genuinely cared for the guinea pigs welfare etc.
So first I would say try a rescue- I know that though I haven't got my current pigs from one my next piggies will definitely come from a rescue and I have rescued other pets before and know how loving etc they can be.