This is the PEI I ordered - 12x12x.06":

http://www.amazon.com/Polyetherimide...ilpage_o03_s00

and to hold it to the build plate:

http://www.amazon.com/TapeCase-squar...ilpage_o03_s00

Score the PEI with a razor knife, then break worked well for me. This is enough PEI to do the bed two times. The 3M tape is kind of a pain to work with, but there are videos on YouTube with details how to handle.

I heard from somewhere the PEI from McMaster-Carr might be higher quality. Making it removable would be ideal. Please share if you find a good solution.