I don't know of one, but want the same thing. Let me know if you find a workable, affordable alternative.

Currently I use my mulching rotary mower to do this. I lay down the material in a channel made from the side of my garage and a long piece of plywood. I then pass the mower over the material. I does a decent job, however collection the munched biomass is a hassle. The ideal setup in my mind would be a small unit attached to the side of my garage with it's discharge chute set a few feet off the ground (directly into a wheelbarrow perhaps.) This could be hand, gas or AC powered.
I am toying with the idea of a oldfashioned meatgrinder. It uses a simple conveter screw to force material through a cutting tool. This would have lots of limitations, but you get the idea... maybe a larger screw powered by a motor. Safety would be important of course.

I've seen discussions on the Mulch and the Tools forums about using a leaf blower with a vacuum attachment (or a mower with a vacuum, for that matter). Supposedly the leaf blower will do a decent job with small quantities of leaves, as long as they're not wet. I guess the unit sucks in the material and shreds or chops it. For your purposes, that might fill the bill? One low-end model that was mentioned several times cost about $40-50.

The main complaints I've read about using a leaf blower are 1) that wet material jams the unit, and 2) that if you try to do BIG quantities of leaves, a critical part on a lot of popular models wears out pretty quickly.

If you're talking pretty small batches of stuff, I wonder if you could nab a used restaurant-sized, variable-speed blender??! One with blades that can be sharpened easily every so often? Just a thought!

I've used the mower method; I clear a spot, heap the leaves, set up a piece of plywood so the debris isn't blown halfway across the yard, go over the leaves a couple of times, rake into a pile and use a flat or snow shovel to put into the wheelbarrow.
Another method is to put the shredee into a barrel and use your weedeater (as shred-or) to grind it up - kinda like a giant mixer.