Which state are you and can you add that to your details, please? That makes it a lot easier for us to give you advice without having to ask a lot of questions first.
Here is how to:
http://www.theguineapigforum.co.uk/...cation-and-creating-an-avatar-picture.107444/
If you can, going to a good standard rescue is preferable because their piggies have been quarantined/health checked and treated, gone through a pregnancy watch (or have been gender separated properly if rescue born to a pregnant incoming sow) and have been paired up carefully - but if you can't, then you can't! The vast majority of pet shop piggies are fine, or they wouldn't have been in business for that long due to complaints.
These are the things that you have to be aware of or need to look out for when buying piggies:
If you get two young boars, you have to be aware that they are going through a hormonal teenage time between 4-14 months and that there is a certain risk that their personalities can clash if they turn out to be both fairly dominant. By far not all boar pairs will fall out though!
With sows there is a low risk that they can be pregnant when you buy them if there has been sexing mistake. The risk seems to be higher in the UK, as chain shop piggies there are imported from breeding farms on the Continent and not separated during a several days long transport to various shops even if they have been gender separated in the shop itself.
Make sure that any piggies you buy don't have gunky/crusty eyes or noses, are not itchy or have any bald spots. These are the symptoms for respiratory infection, mange mites or ringworm, which are the three rather infectious opportunists that can hit stressed little shop piggies. They can also appear up to two weeks after you bought them (incubation period).
Here is a list of good standard US rescues that we can recommend (we are currently in the process of integrating it into our own rescue locator in cooperation with Guinea Lynx):
http://www.guinealynx.info/rescue_organizations.html