Dulce de Leche is pretty much cooked sweetened condensed milk. It is like caramel but not exactly. There are several different ways to make it from boiling a un-opened can in water to pouring it into a pan and cooking it. Search some past threads and you will get all kinds of ways to make it.
Since I have found it in the can I just buy that since it is already ready to use. No cooking or cooling time involved.
I found it at Walmart in the Spanish/Mexican food section.
This is what the cake looks like: http://www.mexgrocer.com/4599.html
I use this all the time for filling in cakes and everyone loves it.
Jodie, I printed out your caramel recipe. I plan to try it sometime as well.