Guinea pigs (Cavia porcellus) are small rodents, they were domesticated in South America at least 5-6000 years ago. According to the genetic tests their closest known relatives are mounted cavies (Cavia tschudii). 

They belong to the Caviidae family - just like the lesser and great maras, and are close to the capybaras as well. These rodents evolved in a unique way to fill the ecologic role that is generally filled by grazing large mammals with hooves, since the latter were not widely present in the continent.

This evolution made them different from other rodents. Thanks to the grass diet, their molars are continiously growing. Since they live in an open, grassy environment, their legs are capable of proper running. And for the same reason, their 2-4 newborns are well-developed, miniature adults that requires minimal nursing.

The domestication started in current Peru, Ecuador or Bolivia, they were kept as a source of meat. The first specimens arrived to Europe at the 16th century, in the beginning they were kept as expensive, exotic pets of certain nobles. Medical researches discovered guinea pigs in the 17th century.
Selective breeding began in England, currently the number of guinea pig breeds are over a dozen.

Purebred or mixed?

They are both the same species, but look differently. Mixed animals are generally from pet shops or backyard breeders, their genetic history is unknown. Currently it is a widespread opinion, that the genetics of mixed animals are usually poor. The main reason behind this is uncontrolled inbreeding.
Purebred animals have verifiable background, and the responsible breeders remove the animals with known genetic problems from the lines. They follow a certain standard look.
Obviously, this isn't true for all the breeders. The ones that breed for profit or don't have the knowledge are different. Thus, it is extremely important to check the source of your future animal.

Breeding cavies with high quality forage, proper vet care, without the exploitation of sows or getting rid of the old animals cannot be profitable. Not even close to that. It's a hobby with massive net loss. This is why the number of suggested breeders are so low.

Where to get cavies from?

It can depend on your preference. What is crucial:

* Do not buy in pet shops! The ancestors are unknown, they have limited knowledge about cavies and generally they cannot sex the animals, so premature pregnancies are very common. Even if you feel sorry for the piglets, please do not try to save them. Every sale is a positive reassurance, and the supply is endless.

* Do not buy from backyard breeders! Breeding means much more than let two animals to reproduce. Purebred animals with proper documentation is just a minimal requirement, but even that is something that 'breeders' cannot accomplish most of the time. The number of caviaries with purebred stock is about half a dozen only. Backyard breeders exploit their animals and doesn't have the skills. Do not support them by buying their products.

The mature decision is to adopt from animal shelters, accidental home litters or to buy from those few cavy fanciers.