A friend is wondering what to buy.
I use all-copper "Chore Boy" brand pot scrubbers. Start unwinding the copper "yarn" and wind it around a worn bronze brush to make a nice tight fit in the bore. After cleaning the carbon fouling with solvent, run the Chore Boy brush through a few times to clean out the lead.
Also, if he is getting a lot of lead fouling, he probably needs to adjust the load (if handloading) or try another brand if using factory. Proper sizing of the bullet to the bore is crucial, as is bullet hardness.