I got one from Mr Portugal. You need to strech it from end to end and staple it then strech it from side to side were the sear bends upwards keep streching from side to side and work your way towards the front and then the back, do it indoors were it is warm. If you get creases undo the staples and try again.
picked up seat from Upholsterer excellent job 10 quid--bike looks like brandnew--mr portugal recomended--the guy in the shop sold the same material for 10 pound a meter