Ok. I think it may be of benefit to approach this the other way around if you need to do it in future.
Xtradecal and Modeldecal are, I would say, the go to sites for RAF ‘support’ decals such as roundels and letters.
Modeldecal set the standard but are sadly no longer in business, although several of their 100+ sheets are still available from Hannants.
Xtradecal are still in business (so to speak) as the are part of Hannants, and they fill the gap left by Modeldecal.
Other companies such as Model Alliance and Printscale also provide quality decals but generally for complete markings rather than the specialist roundels and letters.
That said, I’ve just remembered I do have letters and roundels sheets from Kits World.
Hope this helps.