Paul,
If you haven't already found something, go to the Sonotube Projects Links at the top of the DIY section of HTF. On that page, near the bottom, there are several MP3 and/or .wav files of freq. sweeps that you could use.
I believe there are also free downloadable tone generators that might serve your purpose. I don't know much about these, but you could do a search for "tone generator" in the DIY section and probably come up with something useful.
JKS