Please can anyone show me the link for non-inductive resistors ( in series) which can work for MG-20 and with all different values?

I searched in Newark, Mouser and Digikey but couldn't locate the resistor for MG-20.

Also do I need to specify wattage or any other specifications and what maximum value i can use without harming the speakers?

I send email to Magnepan but couldn't get much help.