I escaped L.A. two years ago, cant speak to SD other then there are a couple right friendly Muzzleloader groups down that way.
As others stated; within Los Angeles county, Angels Firing Range is your best bet. BP friendly, even had some women show up one day with a Canon (that was a Blast!).
Just north in Canyon County (Santa Clarita) you are out of LA county and a friend we could shoot right off his back porch.
Dont know about today, but two year ago Angels Range always had Goex on the shelf (unlike Nevada were I now have to mail order).
However, CHECK LOCAL LAWS! Different counties have different laws on How Much Powder you can own!! Los Angeles was "1 pound per person"!
I personally will NEVER set foot back across the border...but Yes, please stop by Angels Range and tell them Mad L says "Howdy!" (Angels Range is where my wife and I first met
!)