The Xtal Set Society sells a range of crystal kits:
http://www.midnightscience.com/kits.html
You can also buy a basic kit from Antique Electronic Supply:
http://www.tubesandmore.com/
If this will be a group project, you might consider building a kit yourself beforehand, and trying it out in the location where your group will be working. It might be disappointing if everyone completes their project and you hear a chorus of "Can you hear anything?" "I can't hear anything!"
Phil Nelson