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)