If you need a melaya leff itself, just buy three and a half or four metres of stretchy black fabric (stretch is better) and if you have time, decorate. Mine is made from a ribbed knit that my teacher got off the bargain table - anything like that will do - and it has silver paillettes three deep around the edges and down the middle. Voila, and people always coo and ooh and ahh over it. And yes, any hoochy mama dress will do.
Here is me and my friend with ours:
http://pics.livejournal.com/suzycat/pic/000597e6/
Obviously our dresses could be hoochier. I have a much hoochier one now.