50-70$ will give your students a good if not excellent used analog scope such as suggested from numerous members in here
Other than eBay, less obvious sources for getting a scope on the cheap would be your local HAM (radio amateur) club, there is always one in your city, look them up (they're really friendly and love new faces). Another way to get some cheap scopes, is to visit the various electronics repairshops, service dept. etc. Ask nicely, perhaps bring a free pizza to the overworked technicians, and who knows? Maybe you'll end up with a Scope for the price of a decent pizza slice. (I KID YOU NOT, I've heard friends of mine who have done this, and even gotten a free Spectrum Analyzer, albeit old...but working).
Yet another source is the various tech-schools out there, they have old surplus equipment too, one of my friends also got a serious stash of scopes from them, perfectly legal. You could even look up military surplus sales, they often sell truckloads of much better stuff, some people make a killing buying pallets of Scopes, analyzers, bench multimeters, solder stations and much more from the military auctions, and re-sell them for seriously high prices on eBay.
A few things you may want to know about old scopes though, is that they are FRAGILE. Scopes around 20mhz are useful for low-end digital experiments and standard old audio & CCTV repair and experimentation (enough to teach you!) A 100 mhz scope throws you into the digital era, you don't need much more than that. When you find one (beggars can't be choosers, but if you pay a little...) then you may want to check that all the knobs are okay (yes, you can lube them yourself, but check for broken plastic bits, if it breaks - stay away), Good strong CRT (no hefty burn-ins or weak display), also look for the famous LOST TRACE (this means loose parts inside, again...stay away unless you know what you're doing).
A couple of good scope probes can be as expensive as the instrument itself, you may actually want to purchase those from China, they're okay...and cheap. Test leads are the only thing I recommend people to purchase new, because they take the most beating.