A backbore is the section between the throat and shank of the mouthpiece of a brass instrument.
Try ebay. That is where I found one. Also, depending on how much has been split back, the barrel can be shortened, recrowned and either jug-choked by backboring or even interchangeable chokes can be installed. sales@countrygunsmith.net
Depending on what the choke constriction is, it will only get so tight. Have a gunsmith lengthen the forcing cone and polish the bore. If you need it tighter, you will have to consider backboring the barrel, or having screw-in chokes installed. sales@countrygunsmith.net