The RAF shirts are really nasty and cheap (just shy of a fiver last time I bought one) - but they are about 80% polyester. There is one that is the same pattern as the RAF one and roughly the same shade by KustomKit - it’s about twice the price and about 70% cotton.
https://www.kustomkit.com/product/co...-short-sleeve/
Otherwise you would need to get a tailor to knock you some up. Here is a company in London that does 100% FAD cotton shirts for the Army - £54 off the peg or £75 made to measure:
Military Shirts
The only other way to get this done is to get enough of you together and put in a THREADS report via clothing stores compalining about the current shirts. Then it will go to the 6 monthly Clothing Committee to consider - you would need at least a Squadron’s worth of reports to make it viable for consideration.