k750 firmware is easy to find!!!
you dont need to find your exact firmware you just need to find a matching main + fs firmware
However w810i firmware is better so i tihnk yoy should flash to that
its up to you here are firmwares from both phones you can flash with either, as long as your phone is cid 49 or 36. any flashing program will tell you whether it is or not.
k750i firmware only for cid cid49 and 36
Download
or if you want w810i firmware look on here! only if your phones is CID49(i think you might be able to upgrade to cid49 with xs++ or sony ericson update service)
Download
you alson need xs++ if you havent got it
http://www.se-nse.net/gbrooks3/XS++/download.php
thats all you need then you can flash, just make sure u select a customization pack after flashinng firmwares otherwise the phone wont start.