I don't. I don't even know what a dummy plug is.
I'm going to guess that it'd trick the computer into thinking it was hooked up to a monitor.
Where could I get one? Would a DVI>VGA converter work?
Yes, a dummy plug tricks the card into thinking a monitor is attached, and in turn tells windows to load the drivers. Without a monitor attached Windows just ignores the card.
They are easy to make. If you don't have a VGA output on your cards, you will need a DVI->VGA adapter, but that alone will not do it. You also need some resisters.
Run down to Radioshack (or any electronics/radio store) and pick up a pack of 68-ohm resistors. At Radioshack the part number is 271-1106, for a pack of 5. You will need 3 resistors per card.
The instructors I followed are located at:
http://www.overclock.net/folding-home-guides-tutorials/384733-30-second-dummy-plug.htmlAnd are pretty simple. Just plug the resistors into certain pins on the VGA port, and you're good.
EDIT: yeah, $20 on ebay for 1 is insane. I'm assuming you have DVI->VGA adapters laying around, I know I did. If thats the case, you can make them for less than $2 total.