Try Angler's Workshop. I was looking at their catalog this morning, and I think they have what you want. It's the sliding band seat with a slightly swollen butt to hold the bands in place. You combine it with the handle you want to get the uplocking function. It's shown as FG 15 in the section on rear grips on page 146 of the catalog.
However, having said that, I' would probably do as suggested by others and do it myself.
I'm in the process of rebuilding my first hand-built fly rod, which has a sliding band reel seat, and my thin the butt to conform to this shape.