I purchased a MK2A Heated Bed for my RapMan 3.2. What should I use for a power supply? I am going to mount it under a pane of 3mm glass.

I use these: [www.ebay.com]
But a PC ATX supply would work just as well, as long as it can put out at least 15A on the 12V rail.

Yep but you cant just use a PC supply just like that, you need to short some pins so it will start and sometimes you need to introduce a load so the voltage will be stable. If you have the money get a nice LED power supply from ebay. If you want it faster I have also seen them on Amazon but they cost a bit more.

A 12volt 30a supply should be able to supply both your printer and the heat bed.