nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpqyrlmnr0y2ch5ycn38j2navyduf5q6tqyd0led3p7xhwmzwcrlsaqg4kt9v (nprofile…kt9v) The bed itself is 4mm borosilicate from http://www.tridprinting.com/Heated-Bed-Components/ with a magnetic adhesive sheet on top and cut sheet metal, but there are plenty of round pre-made spring steel bed surfaces you can get too.
Delta particularly needs a flat and low-warping bed to calibrate right, so you want the borosilicate. You can also get that on Ali.