my question is, are you wanting a backrest for the driver or passenger?
for the passenger there are several options. as pointed out by Purveyor of Crooked Facts, Garauld creats a thing of beauty. yamaha, givi, shad topboxes can be purchased and a pad added. sargent seats makes one too but you have to buy their saddle to mount it to.
for the driver there are a few also, a bit more pricey. i believe russel daylong has an option for a drivers backrest, but as with the saegent you have to buy their sets.
i've been intrigued by the drivers backrest made by bakupusa
https://www.bakupusa.com/YAMAHA-FJR.html . these things are very pricey unless you can find a used one.
i found one on craigslist a few weeks ago and installed it this past weekend. tomorrow will be the shakedown run with it.
i own the yamaha topbox with a pad for her riding pleasure. i fit just right in her umm, lap, when we are riding two up.
good luck finding what you want, be patient, deals are out there.
cadman