I would also suggest eBay. I'm always seeing all sorts of parts there. Also, you have a beautiful Vulcan 750. I also love your storage/luggage ystem you got going on there. 
I was thinking the SAME THING, that bag looks like a very comfortable backrest! I may consider doing the same for my Vulcan one day.Yes, that bag looks like a nice backrest.
When I began building my bike I set up an eBay search, sorted by newly listed, and I checked it every day. That's the best way to catch parts and you rarely have to look beyond the first page to see anything new.
If I'm looking for a specific part I set the search for lowest price and shipping.
No extended backrests on there so far today.
This is a beautiful Vulcan 750.