Check local salvage yards for new arrivals.
Car-Part.com--Used Auto Parts Market can help you search for a vehicle. You could also get another vehicle of similar year. Most 90's era vehicles are simple in design as they weren't OBDII compliant until mid decade; when things went even more electronically controlled.
Depending on if you want to tear it down and then rebuild the engine with the hopes of it running after, you'll need a few parts to do so, mainly gaskets and extra fluids. Working on smaller engines is a much easier approach for a beginner. A lot of folks want to step right on over to rebuild V8's and such, which are more complex and involved.
The thing to do is once you find a vehicle, obtain a Haynes or Chilton repair manual for around $20, for that particular vehicle. This will aid tremendously and can help troubleshoot anything you might encounter.
Again, local salvage yards would be my first look. You might get lucky and find a car that runs, but is branded and cannot be put back on the road. Those types can be had for cheap. Another is to check ALL local classifieds, even the HillBillyTrader and such. If it's out there, look it up in your area. With the economy the way it is some people may be hard up to get rid of their older cars, especially if they have several then you can negotiate a fairly good deal. I'd expect to pay anywhere from $200-2000 depending on the condition, where you bought it and who you bought it from, and the miles.
Be sure to start it and make sure it runs smoothly with no abnormal noises before you buy. If the engine has some problems due to lack of maintenance or high mileage, it might be harder to get it running upon reassembly.
If you have enough money to do so, I'd shop around for a late model focus. They're a dime a dozen for the most part, and it would make more sense if you want to learn about engines, to tear down and rebuild a newer car. You should be able to find a 2000 Focus for a very low cost. Keep in mind there were two different engines, both 2.0L in size. A 2V 110hp and a 4V 130hp (Zetec). The Zetec is more complex and is on par with even today's standards. The current 2009 Focus runs the similar Zetec engine with some tweaks to improve efficiency and power (136hp), but the overall design is relatively the same as it was nine years ago.