Queenyuki 3D tech has nice products/prices

Aluminum support from IMS

Alternative to PEI

Also lokbuild is supposed to be the best but 16x16inch is not available http://www.lokbuild.com/

PEI sheet for 15.75 x 11.81 sheet is $84 from Professional Plastics but you can find it cheaper

This is an adhesive one

This one is just a nonadhesive sheet

cube([400,300,3]); inches 15.75 x 11.81