Andis clippers are pretty good clippers. I am not sure if you can find them at a petstore though. But you can find it online at jbpet.com
If price isn't a concern, I think the Andis Super 2-speed will work for you. I haven't tried it myself but it sounds go for what you are needing it for. I have a cocker also and she has super thick hair too that takes forever to cut with crappy clippers. My good ones broke down so right now I am using a cheap one until I can save the money for the other ones.
www.dog.com also carries some Andis clippers.