I have experience in changing Large systems that do not want to change...If you want to get Petland, Petsmart, Petco, and other large Corporate pet stores to change the only way you really can is by effecting there image and sales. I personally avoid PETA simply because they end to draw bad press on themselves and repulse many people they are trying to reach with some of the tactics used. If you want to change one of these companies it is possible and not as difficult as you think it might be. Internet petitions and all are nice but the Pet companies know that these serve to let people "feel" like they have helped and done something and that it usually ends there and there is no adverse effect on them. You'll never get pet stores to stop selling pets and pet supplies and to try is futile. You have to approach the issue from a perspective that seems to give them a benefit in doing what you desire. I will describe an example of what I mean (this is an example only):
Assume that I know someone or a company that could mass produce C&C cages for a very good price.
Assume my goal is to get the store/chain Widget Pets to improve their treatment of cavies overall.
Last Assumption

Assume that there is myself and a group of people involved and dedicated to this task both at the local stores and nationally if need be.
I would arrange to meet with Widget Pets PR person or whatever they may call that person for the company. In this meeting I would inform them that my goal is to improve their treatment of cavies overall. I would also inform them that I am not against them or trying to cause a conflict with them, rather, I think that if they listen I can actually increase their sales/profits and increase their image/PR which would also increase their sales/profits. The alternative would be negative publicity for the company and an ongoing campaign to educate the public to boycott the company (ranging from informational pickets to television and newspaper coverage etc) but that we are trying to avoid any conflict at all.
By offering to increase the profit and image of the company you will probably get their attention and they will at least listen to see if you actually can as almost every large entity really really dislikes bad publicity especially over a period of time and people are really sensitive to pets so they will be willing to at least listen at first.
Then I explain how this will be beneficial for them.
1). We want to provide (free to the store) informational pamphlets about the piggies for the store to post and hand out.
2). We educate the staff (and in the pamphlet) that piggies really need to have a larger cage and should be bought in same sex pairs and they tell customers this, resulting in more piggies being bought along with bigger cages being bought. (and yes I know never buy a piggie from a store but people still do and I'll get to this also)
3). The cages would be the C&C ones I mentioned above so they would be Larger then the other cages and would cost less to the customer and would cost less to the Pet store and therefore would increase $$ when selling it and the piggies would have better homes.
4). Along with the bigger cages would be the proper type of bedding sales and bigger cages use MORE bedding to cover it so that would increase the bedding sales....(Can you see how this is going?? Your appealing to what they want to get what you want)....
5). Also it could be pointed out that once the Widget company has a reputation nationwide of being a very caring and sensitive company with their piggies then they could also do (and some pet places where I live do this) rescue piggies and not have to pay for breeders as much or at all depending on the local rescue association. This would reduce the use of breeders and increase profit again for them.
That is just an example but it is also real as it could be used for real and could work for real. The problem is many people want these places to immediately stop XXX whatever XXX may be, breeders, cages, ect...but the real world does not operate that way so no improvements get made at all. I dislike petland to death because they just have all the piggies mixed and many are pregnant and they have no clue what they are doing other than to sell them and no understanding of the animal at all. My piggies came from a pet store (cept the baby, the one pigie was pregnant when we got it) and I would adopt but didnt think about it until AFTER I got them and started doing research into whats best for them. This I would say is very common and finding out AFTER buying from the petstore is to late. So saying not to buy from one is a good thing but really does not do much in reality for this and other reasons. But if you followed the example above you would slowly over time (in small steps) change 1 store/company to doing much better practices and if this is successful others would follow because if it works for this company then it would work for them. The problem is that this would not happen overnight and could take months or years to accomplish this. The last project I worked on to change a large entity like this took me 8 years to finally get changes to occur.
Anyways I am rambling and should stop

But if you are serious about changing things then just know that it can be done and that 1 person can make a difference and change them if you have patience, a clear goal, and and understanding for ALL perspectives involved so you can get to your goal in the end.