I think what you need is a GaAs wafer suppier to provide the "blank" wafer of the size you want (3", 6" and later 8")
Here is a snapshot of the marketplace for GaAs wafers on Alibaba:
http://www.alibaba.com/showroom/gaas-wafer.html
The foundries will need a MBE added to their normal complement of fab equipment. To my knowledge, the MBE is the only requirement (besides the GaAs wafer itself) to make a fab POET-enabled.
From my research, a fab can add a production MBE for a mere $1M.