For the glass bed, look at McMaster Carr for the parts. Here's the links:

http://www.mcmaster.com/#catalog/120/3646/=ubs8gj

Look at part number: 8476K74 which is 9" x 6" 3/16 Borosilicate glass and it's $19.02 per sheet.
Get two of them (one for ABS, one for PLA)
Heat-Resistant Borosilicate Glass

  • Color: Clear
  • Maximum Temperature: 445° F, except tempered pressure-rated discs are 500° F
  • Machine: Use glass-cutting tools
  • Annealed, unless noted

Also known as Pyrex and Schott glass, this glass resists clouding and pitting and is highly chemical and heat resistant. With a low rate of thermal expansion, it also stays dimensionally stable during changes in temperature. Meets MIL-G-47033.

1/8" to 1/4" thick sheets have cut edges. 3/8" to 3/4" thick sheets have groundedges. Width and length tolerances are ±0.062". Thickness tolerance is±0.008" for 1/8" to 1/4" thick sheets, ±0.011" for 3/8" thick sheets, and ±0.019"for 5/8" and 3/4" thick sheets.




Now go get the following from Thingiverse: http://www.thingiverse.com/thing:278540/#files which are the clips

Then get the Z-axis stops and adjust so your heads don't slam into the glass.
For adjusting the Z-limit switch, it's easier to add a clip (http://www.thingiverse.com/thing:43783) than to replace the whole back plate.