Folks, it is not illegal to purchase a 14" barrel for a shotgun. As long as the man has a registered short-barrelled shotgun or AOW, it is perfectly legal for him to own the barrel. (Point of fact, mere ownership of a 14" barrel does not, in and of itself, constitute a crime. It is only a criminal act if the person owns the barrel and a shotgun, of the type for which the bbl is intended or will function with, that is not registered as an SBS or AOW.)
It doesn't do the firearms community any good if you guys jump in and tell someone that they are trying to commit an illegal act if, in fact, they are not. I would suggest you go to the NFA forums and become versed in the intricacies of NFA34, GCA68, and FOPA86 before you offer any further bad advice.
To answer your question, find a local Remington Law Enforcement Distributor
. Contact them, provide them with a copy of your Form 1 or 4, and they'll be able to hook you up. When I spoke to the local guy here, he told me a factory 14" bbl would run me $175, so I'd expect they'd cost pretty much the same there in TX. (I've got a Form 4 submitted for my 870 AOW and a Form 1 for an 870 SBS right now.)