I've used Pit Replica, straight from them on ebay, used them on CR500RE, KX250A5 x 2, RM125B, RM80B and they even custom-made a cover for a KX250A5 with KDX printed on the seat for my A5 with a KDX400 engine fitted.
Next time I order one I will ask for an extra 5-10mm all the way around, they all were a bastard to fit and pulled a little too tight in the corners. Apart from that they are great.