Target has some. They have the ones that are just the tape and book for 6.99 or for 9.99 you can get some in a little plastic case that have the book, tape, and cd. The cd plays in the cd player or the computer. When it is in the computer you can read along and watch the story play on the screen. I like the fact that they come in a hard case, kinda like a video case, that keeps it all together and travels in the car. We have Little Mermaid and Pooh, but it shows Atlantis, Toy Story 2, Lion King, and Emperor's New Groove on the back of the box.
If you are trying to get cheap ones, how about making them your self. I have bought cassette tapes at the thrift stores, placed tape over the holes on the top so I could record on them, and recorded them myself. Kinda like a bedtime story. Sometimes when I get creative, I will make sound effects with things in the house. Ring a bell when it's time to turn the page and it as good as store bought. (My girls like them better because it's mommy.) I use a black marker to color in the writing on the tape and use stickers or make a sticker on the computer to put on the tape with the book name.