Wojtek, the 'ALPS' printer has the extra white cartridge, in order to be able to print white - they cost a fortune though!!
There are 1/48th scale decal sheets available for RAF codes, in grey, dark red, black, sky and white. These are normally a selection of 4 or 5 of each letter and number, in the various sizes and styles. Not sure about your local stockist, but if you have a look at the Hannant's web-site (UK based, and they do ship overseas), you'll find some there. Just 'Google' Hannant's and you'll find them. Enter the details in their search boxes, clicking on 'Aircraft Decals', and the scale, and you should find the whole list, which will be illustrated where appropriate. Check that the codes were white though, as the light grey were used during certain periods.
Hope this helps,
Terry.