I get pigments & medium mail order from Brodie & Middleton http://www.brodies.net/ and mix them with an acrylic copolymer by a company called 'Spectrum'. The fabric that is used to paint on is called Trevira, its great because its inheritantly fire-proof, light-weight and rot-proof.
just had a look for this too.
If you go to the Brodie & Middleton link, then to the price list, under scenic and display fabrics you will find trevira and it costs about Â£20.40 for under 25 metres
Hope this helps