Search the archive (select search above) for foam seats info.
Basically what I did with some help was fill a heavy duty plastic bag with a modest amount of gunk, seal it with tape, sit on it whilst it sets to your shape and that's it. Make sure you cover the cockpit with protective plastic as this stuff does not come off if it attaches itself to inappropriate surfaces. It don't look pretty but if only for trackdays who cares
You can buy mail order from these guys but I repeat Imay well have sufficient for your needs.
Jan
http://www.cfsnet.co.uk/acatalog/CFS_Catalogue_FOAMS_14.html