Kubota L3710, Ford 5600, Case MB4/94, Kubota B6200
Manufacturers like Saf-T-Cab sell through dealers. Their URL is http://www.saftcab.com/stc.htm. Call them and see if they make one for your tractor. Since Yanmar made some of the JD tractors, a roll bar for one of the JD models may also fit your tractor.
The key factor is going to be the axle cross section and the space available between the seat and the fenders. If your tractor matches another brand and the tractor weights are similar you might have a match in the ROPS made for the other brand.
Saf-T-Cab will NOT provide any ROPS or devices for greymarket equipment. I have tried a couple of local backyard welders and they would not fabricate me a rops due to liability issues.
The only place I know that commerically sells a ROPS for a grey yanmar is homestead tractor http://www.hstractor.com/. They call it a roll bar to most likley get around the ROPS certfication requirements. They have patterns for several yanmars. I know they have ROPS patterns for 1300, 1400, 1500, but not sure about 1600. I know they didnt have one for my 1810 last year but that was a pretty new tractor to the US greymarket.
It might be possible to adapt a John Deere ROPS from a 650 or a 750. You might be able to find a used ROPS at a dismantler like www.wengers.com. I have heard but never checked that JD sells ROPS at dealer cost but I don't know if you have to actually have a JD to qualify.