Hi Boonerball, you don't say where you are based, so here is a few generic choices.
There are a few mfrs that do
Search ebay for: Eduard 1/48 seatbelts, you are looking for their RAF late pattern colour etch set.
or HGW: search for: HGW 1/48 Micro Textile Seatbelt Set 148005 Spitfire
Other search engines & auction sites are available, or try Hannants.co.uk
ditto search for Eduard or Montex spitfire paint masks, most will be for wheels or canopies,
but I believe someone has produced camo masks, but if you search the forums, you will find tutorials on how to: