Any hardware or home store should have them for around $8. They're very cheap and a basic bowmaking tool. Though I prefer my #49 and #50 Nicholson rasps, I use a 4:1 rasp (especially the file side) quiet a bit for shaping grips and tips. Almost a decade ago, I used just a 4:1 rasp and a pocketknife to tiller my first osage bow from a roughed out blank that I traded for.
An ideal list of tools for a beginner, IMO, would include a vise, drawknife, hatchet, long handled (12") sureform rasp, 4:1 rasp, small chainsaw file, gooseneck and straight scraper (or substitute such as a pocketknife for same), and assorted sandpaper. Of those, the vise is the biggest expense, but combined, you should be able to aquire everything for under $100. The really budget minded can squeeze by with a hatchet, sureform, 4:1 rasp, and pocketknife to tackle everything from tree to finished bow.