From what I found while googling, the old 6950 1gbs might be unlockable (luck is still a big factor in getting one that is unlockable, but it is possible).

Aside from that, reading the mining hardware comparison chart, it does not look like 2GB 6950s are any faster than the 1GB ones, so I'll also say just get the 1gb ones. (You could also search for some old 6950s and try unlocking them if you are feeling lucky)