Pennine outdoor sell a ventile olive green cotton that is cheaper than 'Ventile'. At the moment its going for £10 a yard. Its the material that was used by Sir Edmund Hillary to climb Everest. Ventile is not very wide so you need more of it to make a jacket.
http://www.pennineoutdoor.co.uk/catalogue.asp?cID=1
Sandsnakes
http://www.pennineoutdoor.co.uk/catalogue.asp?cID=1
Sandsnakes