I purchased mine from Lowes. It is a 2 part system. The backing that has adhisive on the back the slot to hold the wire and then a cover.
Link to channel example
The offer 2 widths for different size wires. They do not taper at both ends but do hold paint well.
Here is an image of mine in the front of my room.
They have all the little angles and such for turns and capped ends.