Crosman R1 Barrel Swap

Looking at getting the Crosman DPMS or R1. Probably the R1 as I can change the stock on it, question though,
I know part of my mod list will include a longer barrel ,I would like at least an 18" barrel, with a new, longer handguard. I've seen custom R1's and DPMS's on Ebay, looks pretty sweet. What I would like to know is where to source an aftermarket barrel that will work? Also what about a handguard ? It's all milspec so I guess the handguard could be any milspec style, but I'm not sure where to start with the barrel. Any help is appreciated, thank you!!