Hey Steve,
AMW/ProX sell Cesseroni at the CMASS and MMMSClub launches (lots of others around the country, too). When Robert and Gloria aren't in attendance, Jason covers for them. Contact Robert or Jason ahead of time and they can be sure to have what you want at the field.
I don't know too much about black powder but lots of folks around here get it at the Kittery Trading Post in Maine. Someone else will be able to fill you in on permits, etc. As a pound goes a long way for many people, you may be able to get someone to sell you some at the field. From what I understand, Pyrodex needs to be highly contained to provide the right punch for ejection. Again, many others will be able to give you help on this topic.
If you can get to a launch or a meeting, you'll get all kinds of good info.
Kenn