I'm having the same problem. It's impossible to find java moss in stores. I don't think the lfs wants to sell you plants that will survive forever; they want to sell you ones that can die so you have to come back and buy more...
The stores here do not sell it. In fact, I've never seen it for sale in a store. I did, however, get it "accidentally". I remember a few teeny tiny strands being caught in the base of some other plants I bought.
It sounds silly but I know that the LFS I work in can't get Java Moss. Not on the lists, not if you ask. There is a warped plant sold as "Moss Balls" that is not the same plant and that is it. This is from some of the larger north east suppliers. Go fig.