I have a toilet with "UPC" written on the back/top of the bowl between the tank base and the toilet seat hinge. The toilet was purchased from Sears about 10 years ago and is manufactured by Universal Rundle now a part of Crane. Inside the tank cover are two numbers. The Sears part number (73855731) and (I think) a model number (3445). "UPC" is also cast into the tank cover. The model number is cast into the bottom of the tank too.
Using the Sears part number your can go to their web site and access the on-line parts link "http://www3.sears.com/" to obtain a diagram and parts list for the toilet. You can purchase what you need directly from the Sears web site.
If your toilet does not have a Sears part number try calling Crane/Universal Rundle at (800) 546-5476 to order the parts directly.