Another thought: Unless you're a tight-composition fanatic (always better in theory, especially with a digital image, but not always realistic in practice) you may do fine just by getting whatever next-longer finder you CAN find, and cropping out any excess. In the case of a 75mm lens, this might mean using a 135 finder, which is probably easier to find than a 105.
The one thing you'd want to avoid is using a finder SHORTER than your (75mmx1.53x=114.75mm) 35mm focal length equivalent -- that might lead you to compose thinking objects would be included in the picture, only to find out later they had been cropped out!
Try as I might to be a purist, I almost always find I have to crop my pictures anyway to accommodate the shape of the subject, so I'm able to be fairly rough'n'ready in selecting auxiliary finders to use on my R-D 1. To illustrate this point, I'm attaching a couple of photos from a concert I shot last week; these were with a Canon 85mm f/1.5 screwmount lens on the R-D 1, for which I was using a 135mm Komura brightline finder (quite a nice little finder, incidentally.)
A 135 finder is actually a pretty good match for an 85mm lens on an R-D 1, but I still had to do a lot of cropping: sometimes because of the shape the picture needed (as in the first attachment) and sometimes to cut out dead space. The second attachment, a screenshot of the Camera Raw dialog box, shows how much I had to crop out of the picture of resting dancers, to get rid of all the dead space (couldn't get any closer to the stage); the third attachment shows the finished result.
When you're already forced to frame so imprecisely, I figure, having the EXACT matching auxiliary viewfinder isn't worth too much worry!
(Incidentally, for those who have heard the Canon 85/1.5 is no good, take a look at the final attachment, which is an original-pixel crop. Okay, it's no 75/2 Summicron, but I can live with it...)