Are there any aftermarket barrels available for either or both of these kits?

Jordi Rubio makes the 7.5 cm. Pak 40, with the muzzel brake, for the whole Marder series. However the J.R. brand barrels are in two parts (barrel and muzzel brake) which means trying to get a flush and in-line joint. They've also been around for a while and may look a bit dated. Elefant makes a line of barrels which are newer, but also in two pieces. I assembled mine with the kit barrel and a bit of P.E. and it looks just fine.