There's a couple out there, but the ones that stuck out, to me, when I was hunting for the same thing were the Simms Backsaver and the Fish Pond something-or-another.

Simms has two, there's a thinner one and a wider one. The wider one could pass for a weight lifting style belt, and it looked tremendously helpful, but I was simply too fat for it.

The thinner model and the Fish Pond were about the same thickness, however it was my fattitude that sold me on the Fishpond, being the only one that would cleanly fit around me, waders, and other junk. It wasn't the cheapest, but its gotten me wearing a wading belt at all times, and its helped out my back pain immensesly, plus provided some convienent points to hang stuff (staff, net, flask, etc).

The Fish Pond was available both with and without some accroutements. I bought the model without, and in hindsight, wish I'd dropped the extra $20 or so for the extra pieces for when I want to strip down the amount of junk I carry.

Of course, the best thing I could do was to lose the first 30#. That took me from being able to fish abotu 2 hours at a clip to being able to go all day... The next 120# I've dropped hasn't had the same impact as that first 30#, either. A little really went a long way there.