(people not company's) ... fgpa's in quantity.
Just for the record, using apostrophes do not make a word plural.
Given the complexity of programming an FPGA, I doubt there is a larger market for an Arduino shield than there is for bit coin mining boards.
I'm at a loss to understand why you think you need a FPGA shield. If you have a standalone FPGA board, you can accomplish what you want. A shield would not make programming the FPGA any easier, nor would it magically reduce the cost.
Why do you need a shield to do bit coin mining?