It would be pointless listing the 7950s that can be undervolted for 2 reasons:

1) Pretty much only the original versions of most 7950s can be undervolted.  I have 4 MSI 7950s with the 8/6 connectors instead of the newer 6/6 pin.  The newer ones are locked. Same with Gigabyte and XFX and Sapphire.  I assume Diamond, Powercolor, and any other brand would be the same.  The part numbers are usually the same between older and newer ones.
2) The only decent place to find older ones would be eBay (or here) and by the time you get them in your hands difficulty will have gone up 50%.

The days of GPU mining are nearing an end.  If you have cards then mine away.  It makes no point buying new or even used cards for mining now unless you need them for gaming also.