Ah yes, my former A829 watches now sold off and living in Italy, and the Seiko A829 page I re-created in Photoshop a few years back! I'm glad folks can use them!
The rubber strap is or was formerly known as Seiko ZLM27, should be 20mm. I think Hadley Roma has a "similar" strap,l the buckle isn't shiny stainless steel and the rubber is a little flimsier. But it passes in a pinch!
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.