Yes, I have seen that done on YouTube, using a balloon and calipers or something to measure diameter and calculate volume. I probably will try that but just thought this would be simpler to start. Does the pressure in the resonator change as the balloon deflates?
You didn't mention N2? Is...
I found a supplier. SCI Analytical (http://scicalgas.com/). They will mix whatever you need and ship it. Prices seem reasonable, and they are very hobbiest-friendly.
I'm going to start with a known good mixture and get everything else working, then maybe experiment with my own mixes.