yellow pages will have your local places, check for vinyl graphic / embroidery / print companies. we have a few companies around here that do custom graphics for business vehicles and the prices arent bad. or if you enjoy pain and your wallet is a bit heavy go to halfords, they'll make you some crap ones for a few million squid that arent anything like what you wanted.

If your just wanting a roll of vinyl to make your own check http://www.mdpsupplies.co.uk/ , Kevs just got 2 rolls from them, £7 a roll as well so fairly cheep

if you cut the vinyl down to a size that will fit your printer you can print designs on the paper backing (flip the image first) and then cut out the design with scissors.

stick letters on the rear window and use one of the demister elements as a line to keep it all straight. dont forget to use a spacer to maintain a constant gap between letters

cut out the letters with a scalpel and then put them back in the holes where they came from. Stick some masking tape over the top (non sticky side) then take off the bits you don't want leaving the stickers and the masking tape. remove backing from vinyl and apply. carefully remove masking tape.

if you know anyone that works in a school or anywhere ask them if they got a vinyl cutter, weve got a A2 one in college, very useful

If you still want the custom made route, try:

www.jgdecals.co.uk

He should be more than happy to sort you out if you ring him, he sorted me with custom lettering for my models, but its the same thing!

He provided the website lettering and the JG logo (of course), on this shell: