The newer ones have a few hidden screws and then lift off (there are metal clips on the back that slide through the fiberglass). I believe earlier models were held on with through bolts. Should be easy to figure out if you look up into the gunnel area and see how they are mounted.
You have a couple of options, new ones are manufactured by WISE Corp, in TN
http://www.wiseseats.com/.
They have the patterns for most late models, and could likely fab you a set. Not cheap though.
Another option is to do it yourself. Get some marine foam, some material and cut pt wooden or if you can find it a plastic set of backers. Attach with stainless staples and mount. This methood works well, may have done it and it is much more reasonable cost wise. You may be able to use the existing backers as a template.