Epps sells them, and could port it, do a trigger job or whatever else you wanted before they ship it out.
Unless you have experience with a ported or braked barrel already, I would really recommend you try one before jumping into having it done on your new rifle. The increased muzzle blast is something you should be aware of. I bought a Marlin Guide Gun only AFTER they stopped porting them...as much as I liked the gun otherwise, my previous experience with porting (MagnaPort) kept me away from the original, ported Guide Guns. On the other hand, if you don't mind the noise, finding one of those on the used market might make good sense for you.