Many Bass Pro Shops also stock Goex black powder but one may need to ask for it.
The Apple Valley Gun Club in Victorville, CA is a NMLRA charter member club (National Muzzle Loader Rifle Association). They have a contact person that may be able to help you find musket caps or powder.
And there may be other NMLRA clubs in your area on that list too.
http://www.nmlra.org/clubs.asp?WhichState=CA
Here's a list of SASS (Single Action Shooting Society) affiliated clubs in CA.
A local one may be able to help you find powder and caps.
http://www.sassnet.com/clubs/Clubs_l...ate=California