I got one at Gander Mountain. It is "self inflating" with some sort of foam inside. You crack open the valve and it sucks air in because the foam wants to keep it's original shape. When you sit on it you can crack the valve a little bit and it a adjusts down to where you want it so actually you can have different cushions throughout the day.
Camoflauge, elastic carrying strap with aclip to fasten it to seatsand only $12.
Here is a link to BassPro.
http://www.basspro.com/webapp/wcs/stores/servlet/ProductDisplay?CMID=TOP_SEARCH_GO&langId=-1&searchOption=products&hvarSearchString=3 8-865-017-16+&catalogId=10001&partNumber=91377&s toreId=10151&y=9&cmCat=SEARCHRESULTS&x =17&ddkey=SearchResults