You want a mini-displayport cable OR a thunderbolt cable.My 2009 iMac is the 27'" model. What exact cable would I need ? I do not see a Thunderbolt to MiniDisplay cable listed in the store.
You want a mini-displayport cable OR a thunderbolt cable.
They use the same connector and both carry the displayport signal you need.
(A thunderbolt cable will also carry data, but you don't need that for this and a mini-displayport cable is usually cheaper.)
This is a mini-displayport to mini-displayport that should work (and they have longer ones too, depending on what you need):