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GigDigit.com - Borosilicate Glass Bed - 120mm x 3mm - MP Mini Delta
Another option is Silicone Thermal Pad.
400mm x 205mm x 0.5mm Silicone Thermal Pad for CPU GPU Heatsink
Adding A Glass Bed - 3dprinterwiki.info
This is a pretty good guide on how to add a glass bed using silicone thermal pads. Is is meant for the Wanhao i3 and clones but should give you the idea.