PMC Silver Cores
This wasn't 100% clear, but the PMC cores can be bought from Palmers Metals under Precious Metal Clay
Cleaning
This wasn't covered in the magazine, but this is how I clean them. Soak in warm water for a few minutes to soften the bead release, and I use a soft cloth (old T-Shirt) and wipe the inside of the core and outer flange. I let this dry and then I dip in a silver cleaning solution (bought at most jewellers), dry and polish with a silver cloth!
I don't have a tumbler - someone may be able to advise if it could be tumbled?