Dave has the right approach , what type of shooting, hunting/target, flint/percussion, any "style" you have seen that you liked over others,any concerns withhaving the gun be "replica" quailty in parts used and appearance
there are a lot of builders here and elsewhere who could build whatever you want, but the choice needs to be yours,you can get some imput from the forum on choosing a style with a bit more detail on your part, there are a lot of ML gun types out there from 1650-1860 (ball park dates for flintlock/percusion/ beginning of cartridge era)