They do make 36" pillow inserts if that's what you want, but I'd make my own in order to have it thinner and less rigid than the standard form. I used to make tons of them when I was more into home dec than quilting. Make the form out of muslin, stuff it with polyester fiber from any craft store, and stitch it closed. Remenber to stuff it fuller than you think necessary since it will be used for sitting on the floor and will 'squish' a lot. Stuffing with bean bag pellets would work, too.
I'd definitely make the covering removable so it can be washed separately. Washing pillow forms is tricky at best and a poly filled muslin one would be quick and cheap to replace if needed, much cheaper and quicker than a quilted covering would be.
If you decide to go with a commercial insert...
http://www.wholesalepillowinserts.co...oducts_id=1098
Just a tip: Curve the corners of both the form and the cover to avoid dog ears on the pillow. Also make the form slightly larger than the covering for a 'fuller' fit.
http://playfulstitching.wordpress.co...illow-corners/