Cotton flying suit
Hi Helimo,
I don't know why you want cotton as it would burn like a torch in the event of cockpit fire. (Don't use anything made of polycotton either because that melts onto your skin first - and then burns).
Most flying suits nowadays are made from Nomex II for that very reason.
There is a variety of cotton called 'Proban' that is treated by the manufacturer to reduce permanently its flammability despite washing. You might be able to find a suit made from that, but you might have to trawl the US websites to find it. So far as I know it is no longer used for making suits in the UK since Stormbyrd went out of business.
There are, however, ways of treating ordinary cotton in your own kitchen that are said to reduce its flammability; (I don't know how effective these methods are, or what the fabric is like to wear after being treated, or how frequently such processes would need to be repeated).
The main supplier of flying suits in the UK is Beaufort, but so far as I know all their suits are Nomex.
'Hope this helps.
BP.