I've looked into the same thing for a switch rod and found mostly rosy reviews for the Rainshadow blanks. Can't recall where exactly I found them, just typical google-scouring.
One note, I'd go lighter than you'd think for whatever rod you get. A "5wt spey" throws about the same weight line as a single-handed 10wt, and thus the line is going to hit the water with equal commotion. roughfisher.com: Assault with a deadly weapon