It depends on what size you need. I bought the sets made by Learning Resources and they were way too big for my cake topper project (but the kids love them as toys), but the ones Amazon sells that are described as "polybag" sets (because, you know, they come in a poly bag) were great. Dollar Tree has only had dinosaurs, snakes, and lizards recently, no other animals.
Thanks y'all! Found some at Target yesterday. Our dollar store didn't have any or at least I couldn't find them with all the other crap. Ugh, I hate that store.
Re: Plastic animal figures...
ETA: this is the set I bought, but there are others too. https://www.amazon.com/Wild-Republic-Polybag-African-Animals/dp/B000H70LYG/ref=sr_1_1?ie=UTF8&qid=1412481693&sr=8-1&keywords=Polybag+animals