Oh dear, don't use MDF!!!!! It will just fall apart if it gets wet or if the humidity gets too high.

If you need to use plywood, get a sealer for it so it can't get wet. Birch plywood is the cheapest and should be easy to find.

That being said, I don't think you will have a problem finding melamine. If you can't find it in the lumbar section, check the shelving system section.