I've had a harder and harder time finding BP in Socal. I finally went mailorder. I use Graf & Sons:
You have a $20 Hazmat fee, but Powder is $8/lb. I buy 5lbs at a time and even with the Hazmat fee, it's cheaper than local stuff if you can even find it. They let you mix and match grades.
The cheapest place in the country I think, is Maine Powder House, but they have a 25lb minimum.