The car and its variations are described here. Repro cubes are available here, just look up #3454 and you will find them. The car will work on any uncoupling/unloading track, but not on the uncouple only tracks. You need the extra 2 control rails to operate the car.
The cubes are 1/2". I made my own using a bandsaw and a hardwood such as maple. I added “this side up” and “fragile” stickers and the boxes work very well. Here is a video of my Merchandise car dumping my homemade boxes… http://youtu.be/o7g9-4A0uYU
The merchandise boxes were reproduced by Madison Hardware years ago. Madison ran loads of these boxes. They look identical to the Lionel originals and there should still be a ton of them around. Check the parts dealers and eBay.