I just happened to discover that Target sells Estes model rocket engines--3 for $4.99 (at the store). I only saw the B6-4 and C6-5 sizes available. But they are cheaper than the local hobby shop's prices. I think they well the same ones for close to $9, and Target's package contains recovery wadding as well. I'm not sure why they sell them, though, because there was only one model rocket to be had on the shelves. Here's another source for engines and other rocket supplies.
- National Association of Rocketry
- Fly Rockets
- NASA's Beginner's Guide to Rockets
- Essence's Model Rocketry Reviews and Resources, which has a page listing manufacturer's homepages