Most home improvement stores (lowes, home depot) sell screen kits (mesh, spline, roller) and the metal frame. You can get all this for usually around $20. You'll need a tape measure and a saw of some sort to cut the metal frame to size (chop saw, hack saw, etc). You can buy cheapy hack saws for around $10 that'll get you through a screening job.
edit: This is a super easy thing to do and should be feared only by those who can't read a tape measure at all. Even most tape challenged fellows should be able to manage it.