A very important part of preparing for a guinea pig is finding a vet. If you get a guinea pig and they get ill, you will want to get them seen as soon as possible without having to find a vet, so finding a vet in advance is a good idea! Guinea pigs are considered an exotic pet so finding a vet that can care for them can be hard, but it is important that the vet you pick is capable and knowledgable or it can cause problems. A vet that doesn't know how to care for a guinea pig might still attempt to treat them, and can cause damage or even death!
Your local ASPCA may have a suggestion. Or try calling 5-10 vets who don't practice on exotics to find out who they would recommend for a guinea pig. One veterinarian may stand out.
Check out this site for more advice on finding a suitable vet!