First, try Emakershop. They may have some spares. If they don't, your best option may be to source an aluminium plate, cut it to size, drill holes in the right places using your old heater as a template, and stick a silicone heater on the back. These people [
m.aliexpress.com] will make a silicone heater to your size, voltage and power specifications for a very good price.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].