Anyone know of a source for 'Rubber Sheet'?

Looking to get some rubber sheeting, just rubber...flat...in a sheet form i can cut and use for my fan shroud. to allow air to pass through it at highway speeds and shut while going slow to let the fan do its work. Needs to be fairly thin, 1/8" or so..Anyone have recommendations? I dont need a whole lot. Just maybe a square foot, maybe a tad more...

Thanks! :thumbup: :wav: