I have replaced quite a few stickers with things I make myself. Usually a colour copy of the original works well (I have one of those Playpeople police hats with chequered bands, so I could remove the hatband, scan it and email you a PDF. file). Failing that, it's possible to recreate some designs in software, scale them down and print them- I did this quite successfully with the flags-of-all-nations for the houseboat 3540 recently. Glossy (or Matt if you prefer) self-adhesive photo paper (find it on Amazon!) is pretty good for most PM stickers.