I've made my own, but you can get them here also www.alangrovecomponents.com
The ones I've made are for about 2-3" lower mounting on the passenger side with a long water pump, just enough to get the belt to miss an electric waterpump but not so low as to interfere with the lower radiator hose. A more accurate description would be mine are "mid mount"
Some of the really low drivers side mounts, like the March billet aluminum Low Mount kits for street rods won't clear the front subframe crossmember of a Camaro or Nova, or the steering box of a Chevelle. If you are trying to do this with a short water pump style flat crank pulley, the alternator will be too low and too far out(because the size of the alternator to get it to clear the front of the block) to clear the front subframe.
Do you have one in mind that you have picked out?? Or still looking??