I don't know where you can buy it already slit but if I were needing it I would slit it this way.
I would clamp the tubing to a sheet of plywood. Then make a carrier on wheels to hold a dermal or die grinder. Choose a cutting wheel the thickness I needed, fasten some runners to make sure the cutting wheel was on center of the tubing. I would then run the cutter wheel the distance of the tubing making a shallow cut then lower it a small amount for each pass until it was slit all the way thru.
The most time would be in the setup. Much like most of my tooling jobs I do.