It's pretty easy to make a cabled remote shutter release for most entry-level and mid-range Canon EOS cameras. It's basically just a couple of switches connected to a standard 2.5mm stereo jack.
There are a couple of different ways you can wire it. The set-up described here is one way that seems to be widely used. In that circuit, you have to press both buttons to fire the shutter. The AF button acts like a safety catch - you can't fire the shutter unless you're already pressing the AF button. My circuit is slightly different:
On my remote, the shutter and AF buttons are wired separately, so pressing the shutter button on its own fires the shutter without making the camera refocus. (At least that's how it works on an EOS 350D.)
Here's a view of the inside of my remote. The black button is the AF button, and the red button is the shutter release.
Handy hint: you can get a nice, flexible cable for your remote from a broken PS/2 or USB mouse. You didn't throw that broken mouse away, did you?