The basic code letters can be obtained form a codes sheet, available via Hannant's, from Modeldecal. They are the Dull Red British WW2 Codes, available AKAIK, in 2 or 3 sizes, the ones you need being 48inch by 30 inch, in 172nd scale, set No.82.
Serial number decals should also be available. As for other markings, such as nose art, if any, then a look at the Lancaster decals at both Hannant's and Model Hobbies might turn up something.
Hope this helps.