Instead of adding helper springs, you're really better off replacing the rear
leaf springs. You can get a pair from American Parts Depot
(www.americanpartsdepot.com) for $250. Of course you'll need new u-bolts and
bushings too. In my opinion it's better to just do it right and be done with
it. Espo Springs (www.espo.com) is another good source for suspension and
steering parts, but it looks like they don't carry leaf springs anymore. (Or
at least it's not on their website.)
Todd