I am in Australia, and we have a great Petbarn near us. They only sell pet produce and not live animals. They are the places you need to keep a look out for. If you are in the US, there are some great places you can order online, as I think a lot of people here do. They should be able to point you in the right direction.
I can understand your being upset about the
Cavy you saw. Maybe you should have chatted to the person there and explained the problem with the nails. I would have offered to assist them in cutting them for the sake of the animal, but that is just me.