Engine rebuilding parts

Usually this is the best place to start!
http://www.acmoc.org/bb/showthread.php?492-Cat-Parts-Sources-List
Florin is pretty popular for hard to find parts and surprisingly CAT still carries lots of parts for the U series.

Like Jeff wrote. Cat usually has most of the bearings, seals, and gaskets. Sometimes undersize bearings are not in stock at Cat but searching machinerytrader.com with your part # (there's a Parts Search link on the left side) generally turns up a vendor with them. I usually try Cat first on the basis that if they see the demand, it provides more encouragement to keep stocking them.
If undersize bearings are not available at all, sometimes they can be made e.g. plain bushing-type bearings found in the pony motor main bearings - I've just had some made out of stock, or you can build the journals back up to std (which I've done with my 5U D2 main crank). Lots of options. There are also Cat Classic parts available for some part #s but fitting can sometimes be an issue so I tend not to go in that direction.
Of course used there's a great supply.

OK great news for my old Cats, Will check them out. Thanks

Florin was 50% off of Cat price on a pair of thermostats. They were brand new in a military cosmoline box. Nice to save $90.00.