They're pretty common. The same speedo as the late model TW speedo with the resettable trip meter was standard equipment on other Yamaha models long before it was introduced on a TW. A good source are XT350's and 600's (It may also have been on XT225's and XT550's, but I've never checked). My bike has a, '85 XT600 speedo. Even the lighting pigtail is the same as early TW's.
The drive ratios at the hub are all the same in Yamaha dualsports. A fat 18 is about the same rolling diameter as a 21, and ballpark is about as good as it gets with Japanese speedos.
I don't know how much Yamaha gets for a new speedo these days but I'm pretty sure you could buy a couple of far superior Vapors or the like for the same munny.