Have a look on E-bay: I got a lovely ride-on hi-viz one from "Helen's Hooves and Paws".......
Well worth getting, you'll wonder how on earth you managed without one. I've got a lovely suede-seat saddle with suede knee-rolls which I'm totally precious over, and this last winter my ride-on cover was a godsend as it kept it all nice and dry!
THE essential item in my tack-room TBH.
Alternatively, if you want a nice fleecy/warm one, you can go onto facebook and there's a "Dreamchaser Equestrian" website and they do these.