Hi there! I think I can offer a unique perspective as someone who formerly managed the Pet Care (small animal) department of Petsmart.
The guinea pigs that are sold at pet stores come from massive breeders (think puppy mills for rodents) where the female pigs used for breeding usually die from the lack of special care when they birth a litter, which is very dangerous. The baby guinea pigs arrive at the store in a small cardboard box. I frequently received shipments where 7 or 8 baby pigs were crammed into one small box, covered in feces and in horrible condition. More than half of them had ringworm or URI's, and a lot died. They were in those boxes on a hot truck for a 4-day transit. The company treats them as products, not living beings, and they are given the bare minimum care to keep them alive.
The sick guinea pigs go to Petsmart's own vets, and in my experience, those veterinarians either have no knowledge of guinea pig health or they truly do not care. They prescribe the same toxic steroid for every ailment, and send the poor creatures back into the care of well-meaning but uneducated teenagers. Many of them die of illnesses that are curable. The "healthy" pigs are almost always sold to people who have absolutely no idea what they're doing, and if their new pet dies within two weeks, the company replaces it with a new one who is likely condemned to the same fate. The cages sold at Petsmart are, at biggest, less than half of the bare minimum size recommended by the Humane Society for a single guinea pig.
In my short time as that department's manager, I refused sales to 6 people who I just couldn't bear to send a living thing home with. In spite of the good resumé experience and pay, I quit that job within two months because of the heinous mistreatment of ALL of the animals the store sold.
In short, I cannot tell you enough how truly wicked the large scale, for-profit breeding of living animals as an industry is. From what I saw at Petsmart, I can only imagine what the breeding facilities are like.
If you adopt guinea pigs, the rescue will help you make sure you have all the materials and knowledge to be a successful pig parent! I can't say enough about how wonderful my experience with my local guinea pig rescue was. They helped me set my babies up to have a wonderful life, and taught me more about guinea pig care than I ever learned working at Petsmart. To spend money buying animals from the pet store is to spend money helping an evil industry exploit more piggies, but to adopt a guinea pig is to save two lives: the life of your new piggie, and the life of the piggie that the rescue now has room for. ❤ Hope this helps
