Early 72 cars still used the 71 style vapor tube that's visable in the trunk. If your car has the smaller vapor tank mounted under the car on the shock X-member, you need the newer tank.
My '72 SBD was 8/17/71 and I have only the 4 vents at the front of the tank. I don't have one in the trunk that I'm aware of.
I think I remember seeing that tube. I thought it was on 1970 models, might be wrong.
Like Bullit mentioned, there may only be one source, I think it is Spectre Industries, I think in Canada. I replaced my tank a few years ago, got it from Rock Auto. With the 10% discount it was something in the neighborhood of $155 then add shipping.
There is a difference if you go to the Rock Auto site and March '72 is the dividing line, not sure what the difference might be though. Cost is about $163 plus shipping.
http://www.rockauto.com/catalog/raframecatalog.php