The two largest vendors of educational probeware are Vernier and Pasco:
Not every school has the ability/money to do probeware based labs, however. So, here is a description of how to create a really simple probe that will allow you to see the fluctuations in light caused by alternating current or see the signal from a TV remote control (as well as do other things I'll describe in a future Instructable).
Total cost is less than $5, but you need a computer with an oscilloscope program installed.
Visual Analyser - Free oscilloscope program for Windows. http://www.sillanumsoft.com/
Audacity - Can be used as a recording oscilloscope. http://audacity.sourceforge.net/
Step 1: What do you need?
-Cheap Solar Cell (I bought mine for $3)
-Headphones from the Dollar Store (you could just buy a microphone plug and some wire)
-Alligator Leads (I used red and black, but you can use any color)
-Electrical Tape (heat shrink tubing would be cooler, but I'm lazy and cheap)
Step 2: Destroy the Headphones
The colored strand carries the signal while the other strands are the common ground. You'll need to strip off the insulation. There are two ways of doing this. You can lightly sand the wire, but you risk breaking it, or you can burn it off. The enamel will burn off readily, but if you burn it too long the wire will melt.