Installing tubes is probably the only way to go.
I think your only option with a fixed choke in the 18"-20" lengths would be a Remington. At that, the REM 18" barrels come with an IC constriction, unless it's a special order. Good luck with that buying only one gun.
Shorter, 14" REM Police barrels are made with a Modified constriction for better shot accountability, but good luck finding one of those these days. In fact my 14" is stamped MOD, but measures closer to an IC anyway.
The Camp Defender, if I'm not mistaken has an 18" barrel and sold with a WIC (Winchester Improved Cylinder) and came with 2 additional tubes; Cylinder and Modified chokes.
Mossbergs are Cylinder bore as well, I think, so you're hooped there.
Beretta 1201FP are for the most part CYL at those lengths and maybe IC if you could find one. Benelli and Fabarm, I don't know offhand. What else is there that's worthy.