I haven't taken the plunge/made a choice yet as I'm still in the process of circuit testing (slightly delayed due to hernia op so can't move/bend too quickly or lift much). I'm also still trying to get my head around sensor design. So now my head hurts!!!!

If you do a google/ebay search with a combination of "universal, 4 wire, zirconia, oxygen, o2, lamda, sensor" plenty of cheap options come up. The only problem is knowing if you'll get one with the right power heating element. It's worth noting that the OEM Denso sensor has a wide body (the exposed part) - for the reference air/heating element??
Whereas cheaper "universal" sensors appear to be a lot narrower; so that may also affect their effectiveness/heat up time??? Alot of them do appear to have a less powerful heating element (eg. 6ohms).

For info: It seems like the generic Yamaha code for the sensors is 8592A(-00-00). There's just different prefixes for different applications. Eg. FJR1300 sensor (2D2-8592A-00), XP500 Tmax (5VU-8592A-00) and XT660R/X/Z (1B9-8592A-00)

This German site lists the European models that use "our" O2 sensor which may help in tracking down a "nearly new" one.

However, it's clearly worth eliminating all other possiblities first before replacing your O2 sensor (especially if you have no way of testing it).