For living moss sources, many poison dart frog vendors will carry such -- also terrarium plant suppliers. I've often seen live moss sold at reptile shows/expos.

As far as what moss to use, that will depend upon the growing conditions you plan on supplying. Some research on different mosses should help you there. I have heard (though never tried) that java moss tends to be a highly adaptable moss.

I had good luck with several different types of moss on ebay.

Here is a link that might be useful: ebay