If you're handy, it's really easy to patch a slow leak yourself, even if you're not handy.
Find the leak by pumping up the tire and pouring soapy water over it. The leak should produce a stream of soap bubbles. If the leak comes from the tread area, you can patch it. If it comes from the valve stem (check that, too) get a valve stem tool and tighten that (about a buck). If the leak comes from the sidewall you need a new tire, and if air leaks out around the bead of the tire where it meets the rim, you probably drove on the tire when it was flat, and you need a new one
Patch kits cost about $4, and include a rasp, a pincer looking thing, some sticky black rubber strips, and some rubber cement. Use the rasp to clean the hole and make it more uniform, then pour some rubber cement over the hole (for adhesion and lubrication!). Place the rubber strip in the gap in the pincer device so that it is centered (You may have to open the gap up with a screwdriver), and push the strip into the hole so that it folds over and both ends stick out a little (so that most of the patch is contained in the tire and the hole, and some of it sticks out). Try not to twist it while you're putting it in. Then pull the tool out so that the patch stays in. It may take a few tries. Cut off the excess and there you go.