They sell an 18mm version of it at watch prince, here is the item number:
Bet they could get you one from Hadley Roma. Or go to Hadleys site get their model number and contact global watch band dot com, he is a Hadley dealer and will be able to get you one in 20mm. Good luck. By the way I have one in 18 that I bought quite a few years ago at a Sears it is ready to fall apart now but has lasted a long time, probably 8 years or so.