Guinea pigs love to be in Single Sex Pairs or small groups.
Guinea pigs living on their own, can become very lonely and depressed, some have been known to pass away when on their own, or sometimes after the loss of a cage mate.
Its best to get two brothers or two sisters, they tend to work better in pairs I find, they always have something to do, and they look nice sharing a meal together, eating vegetables together nad waiting for a drink after each other, you always know they're happy.
Of course being in a pair always carries the risk of fighting, but this is quite uncommon, however some boars as they get older may have to be spilt up, and around the age of 3-5 months, they play for dominancy, and can include a few nips, nose offs and humping.
The best place to get an animal from is an animal shelter, however I have bought 5 Guinea Pigs from pet stores and they've all been very healthy. Try to get them in a single sex pair, such as 2 males, or 2 females, otherwise you’re going to wake up one morning to mini ones of your originals. Guinea Pigs are very social animals, they want you to hold them and require a lot of attention to be happy, they love handling and they like getting social with the family, they're an animal that can be held easily, sat on your arm or up on your chest, and most will sit there being happily stroked, they will squeak when they're happy, most do this when in pairs, on their own, they may only squeak when they think there is food.
They can get ill when on their own this is why I suggest getting a pair, when separated from their other friends, they can find moving into a new cage and home stressful, but most owners don't realise it, they will usually sit in the corner a lot and just eat and drink when they come out their bed, because they have nothing to do without a friend and they're lonely.