Missouri Bullet Co. sells .361" dia. 146 grain lead bullets reasonably. But as is the case for most sources, be prepared to wait a few months for delivery. I have a bullet order in to MBC that is about 6 weeks old at present, and still no delivery. Those guys are working 24/7 to fill orders, and still can't keep up.

There is absolutely no good reason that anyone needs to fool around with loading BP or BP substitutes. 3 grains of Unique and a 146 grain lead bullet is a very light and perfectly safe load in any of the old .38 S&W breaktops, assuming that they are not rusted out junkers. As is factory ammo, if you can find it.