Got it from Home Depot, they had it on clearance for a few bucks (they have these sales all the time). It wasn't a custom length, it's not a perfect fit, but you can't tell. I don't remember how I cut it, I think I might have drilled a hole, then used a chisel to open it up like that.
I actually reran the power to a top cabinet outlet myself. Some of the cabinets have a little space in between them (you have to look from underneath, since the trim is all nicely connected), so I ran the wire through that space towards the top of my cabinets, then drilled hole in top of the cabinet that had the outlet (cabinet above the microwave area). I have rope lighting mounted on top of the cabinets as well, so I needed to drill that hole in the top cabinet anyways.
I am thinking of using rope lighting in my stairs area as well, but it is kind of bulky for that application. Since I want to do my hallways too, I figured I would go the LED route for those applications, but it takes some good planning.