Yes, the link I provided is for a cassette but I think somebody makes one for a freewheel too. Or, do what I did when I turned my old Raleigh Wyoming into a single months back, just use the feewheel that was on the wheel. Line up the chain with the most in-line cog and bingo. The cool thing about that method is if you decide to go back to a multi-speed bike, all you have to do is lengthen the chain. If you start messing with the wheel dish or the like then going back is a harder option. Ultimately I took my Wyoming back to a multi-speed bike and it remains so today with no ill effect.