A number of retailers sell out of print decals, a few I would try is www.rollmodels.com www.flightdecs.ca and www.hannants.co.uk.

Another option is there are a lot of generic RAF squadron code and serials sheets out there. Again, unless there was some specific nose art im pretty sure all the BoB markings were standardized so it should be fairly easy to get the codes, serials and roundles. You just need to know the correct size.