Good topic, I've been looking, too. I can't find one that will easily fold up and not take up a lot of space when it's not in use. In the mean time, I bought one of the new Avery dog vests, which has a built-in grab handle. It's made for pulling the dog up into the boat, and it's advertised it as an alternative to a dog ladder.
I put it on my dog when I went hunting yesterday, but can't unfortunately can't tell you how it worked. Let's just say...there wasn't a lot of retrieving that needed to be done.