I bought a piece of die cut vinyl and a set of instructions on ebay for about $20 - it looks OK, not great - and takes some man handling to get all of the wrinkles out and to make it fit the contours of the seat. Not ideal but cheap.
For about $60 you can get a stiched cover from old bike barn and others - since it is stiched and made up of several pieces it should fit the contours of the seat much better - and look better too.
Corbins run $300 and up but include new and better padding.
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Also just to add to this have a heat gun/hair dryer to heat the vinyl up before you strech it and try to conform it to you seat, or if you dont have either,lay it out in the sun and let it get hot. This makes it about 100 times easier to work with.