Buying one from the SPCA is a good thing too. The SPCA are critter rescuers and do good things for critters - heal them, spay or neuter them, help them find good homes.
PETA do suck. Like any extremism.
Our Pound is a No-Kill and they do all the vet check-ups, shots, de-worming, fleaing, etc.
Other than the vets (who may also volunteer for all I know) they run the place with volunteers. Their biggest mandate is getting the population of pet owners to spay/neuter their animals, which every responsible pet owner should be doing in the first place, IMHO.
Then we wouldn't be finding boxes of kittens outside doorways in snowstorms.