Best option for beefing up rear springs?

[zundapman]

I've got a 2006 Chevrolet Colorado extended cab "work truck." The stock springs are really flimsy, a single cubic yard of mulch which had gotten wet a few days before was too much weight for them and flattened the springs out so I bottomed the rear suspension on the dirt road out of the yard.

I need to be able to fill the bed with mulch or wood chips and haul them safely relatively short distances, and may need to leave a load sitting in the truck for a day or two. I need to beef up my suspension obviously, but... What is the best option? I don't care that much if the ride is a bit stiff when I am running empty. I have no experience with air-ride suspension kits designed for use with trailers. I would like to keep cost down. Can I get an "extra leaf" spring helper kit for this truck? If I do, how much weight can I add to the listed capacity? The on-line automotive parts places don't seem to provide this information.