Why would you want one? The reason it didn't sell very well was because the 5D MkII comprehensively out performed it, even though it was three times as expensive!
Hmmm, the 5DII may well outperform the 1DsIII on price, having video capability plus slightly better low light performance. But this viewpoint seems to come from someone who doesn't use 1-Series bodies on a daily basis. For most jobs I'd reach for the 1DsIII. With a 1DsIII in the bag, a 5D MkII, though undeniably good, is always the backup or second body.
Given the highly evolved performance parameters of the 1DsIII, particularly in dynamic shooting situations, it simply delivers a higher percentage of "keepers" than the 5DII. They are both brilliant cameras, you just need to know which one to pull out of the bag for the situation you are shooting in.
Rather than seeing them them as opponents, they are in fact mighty fine companions.