How to turn a pet store around?

The best way to get around them is to make it seem like it is their idea to better the pigs' lives. Write them a letter, enlcose as many leaflets as you can find with lots of links to here and GL.

Here is an example of something you may write which will probably put them off:

"I am a regular shopper at your pet store and I am disappointed in the way you keep the guinea pigs. The cages are too small, they do not get good food and they are being allowed to breed, which is very wrong."

That is basically just throwing accusations at them. Try writing something like this:

"I am a regular shopper at your pet store and I am pleased with the service I get. I do have a couple of ideas for you, if you don't mind me suggesting them to you."

And then carry on with your ideas. In my experiences, angry letters or words don't work. You have to be polite and civil, praise them in whatever way you can, be it about the way they keep their fish, or the prices of the products etc. You have to word it so that they don't feel too criticised. They still may not listen to your points, but it is more likely to have an impact if you talk about how good they are in other aspects. Do you see what I mean?

Writing letters works. The other thing I've done for a small mom 'n pop store here is offered to come in & help them make better cages. Since it was a smaller store, they had more leeway than a large corporation would for this sort of thing, and he was very glad to have the help. Together we made a divided run down the middle of the store with countertop & plexiglass. It was 30" by 10 feet, and one half was for boys & the other for girls. It was set on low shelves, so all of the small-pet food & supplies could be set on the shelves. And he hung (empty) bird cages above it, so little room was lost by having such a large cage in the store.

Not 100% of pets stores are horrible, but it's the bad ones that give all of them the horrible reputation they have today.
Some owners are simply ignorant, and don't know better, but are also open-minded and willing to improve. But, unfortunately, there are those pig-headed owners who would rather ignore you or even cuss you out. You can just do your best.