I'll give my vote for Black Hole Weaponry as well. Standard wait on 204Ruger barrels from them is 10-12wks. Really not a bad turn around, comparatively to any other custom maker that's turning the barrel AFTER you order. Sure, you can buy Shilen or White Oak (bison, wilson, etc) off of the shelf, but many of them cost more than the BHW.
I'd personally recommend the 20" over the 18", otherwise, you might as well look at a 20prac. I have a 22" that barks fire out of the muzzle, cut 4" more off of it and I can't imagine how loud it'd be on the range beside you. Small bores need length to burn powder. The smaller 20prac case ran at a bit lower pressures (less powder) would be more efficient in an 18" barrel, if you really want to go that short.
Keep an eye on BHW's site too, they do RTS (Ready To Ship) barrels in 204R occasionally too. Usually standard profile in black cerakote.