Welcome to the board. Unfortunately most oem parts are hard to come by. You can try
http://www.partzilla.com/ and
http://www.cmsnl.com/honda-cr250r-elsin ... partslist/. Ebay and ebay UK is a good resource for some oem and used parts. VMX has some good items.
http://www.vmxracing.com/To me the 81 CR's are among the best looking bikes ever built. They can be a challenge to ride because of the binary powerband. The 250 may not be as bad as the 125 as it has a bit more power. Do a search on ebay for old dirt bike magazines and motocross action magazines that feature your model. You can pick up some good info from those old articles.
Here is one-
http://www.ebay.com/itm/1981-Honda-CR25 ... e1&vxp=mtr