Hey Andrew
I get most of my parts from a outfit called Boats.net in the US
http://www.boats.net/ they are great to deal with and if you keep your orders small (half a dozen small items) there std postal charge is $8.95 US but you need to enter the part numbers as they dont have diagrams for older models I just use south seattle yamaha to get the part number and search Boats.net for the part.
For hard to get parts try speed and sport or ebay but dont give up on your local yamaha dealer some parts they sell are cheap I got my swing arm chain guard (the big white one) for twenty something dollars. As for the front number plate I restored it with Jif and Mothers plastic polish.
Yz250h the biggest problem is corrosion of the steering shaft and seal surfaces the shaft on mine looked bad when I pulled it out but once it was cleaned up the rust was superficial and the shaft seal surfaces were OK. The big problem is the bottom seal it's like a normal seal turned inside out (seals on the outside to the steering head surface and is pressed on to the steering shaft) and its the steering head tube seal surface that corrodes and leaks I fixed mine by pressing a 42mm speedi sleeve
http://www.alliedbearings.com/mfg_prod/seals/cr_speedi/index.htmlinto the steering head along with a shit load of sealant and it seams to work great Ive air tested it with no leaks but the big test will be when get it going with coolant in it hopefully soon! hope that helps.
