How do I get a reliable car for not much money?

A few details: I don't use my car a huge amount, but I do sometimes drive to campus and to the store. I also take trips to visit family that are a few hours out of town. And I've used my car to take longer trips to keep my sanity in grad school. I prefer driving when I go out at night (especially recently, I've been too sore to bike). I live about 4 miles from campus and bike there sometimes. But I don't have a contingency plan for rain days - the bus takes twice as long and there's no one else around with a car that I know. I think I probably want to have a car, the idea of biking to the store to get paint etc for home improvement projects/groceries etc doesn't sound appealing. I do have a roommate but she relies on her bike primarily, and so has often relied on my having a car for longer transport.