The full Volvo parts catalog for the 740 years 91-92 is available here (big download) -
http://www.volvotips.com/parts-catal...ts-catalog.pdf
On page 11 & 12 of this catalog it will show you how to decode which brake setup you've got - based on the final digit of the 2nd line of the product info section of your VIN plate. It will also have the original Volvo part numbers which may aid your search - the braking section starts around page 578.
If it's just pistons sticking then a rebuild kit will be the best option, it's easy if you've got compressed air to hand to pop out the pistons, clean up the bores, and fit new seals from the rebuild kit.
Just had a quick look and PFS has calipers in stock - you just need to decode your product plate to determine which you need -
https://www.partsforvolvosonline.com...p?cPath=52_511