An easier thing to do would be to purchase only online or from stores that don't sell live animals. Don't forget about stores that sell animals like Petsense, Pet Supplies Plus, Pet Supermarket and the incredibly heinous Petland which uses puppy mill dogs. If you think Petsmart/Co or bad, trying enduring a Petland. Supporting rescues/shelters is the best idea. I'm not fond of purchasing unwanted animals off places like Craigslist, altho I have done this with fish.
I did try to 'boycott' pet stores for a while, after becoming vegan in 2014. The fish hobby is my life so I found myself conflicted after becoming vegan (if you don't know, vegans abolish everything that uses/exploit animals - and in this case, a vegan shouldn't be buying animals)...I didn't buy another fish until 2019 when I got really upset at a Petco. I saw a betta who wasn't doing well and attempted to get a discount on him. They refused the sale and claimed they would treat him. A week later he was on the floor being sold. An employee told me he'd been there at least 2 months (not my worst purchase tho - I bought a betta with a tumor who'd been in a bowl for 6 months and overlooked when there were no bettas left). Out of rage I purchased him at full price, and I don't regret it at all. Since then I've bought maybe 20+ bettas to rehab from poor conditions and kept them or adopted them to other people. I consider my pet store purchases (all fish, mind you) to be acts of kindness.
Unfortunately I can't really boycott anything, when I work at a vet inside a certain chain store and get a nice discount.