I think you've answered your own question with the post title Java moss is pretty easy to grow, as are most mosses to be honest.
Ebay has a lot of mosses, but GreenLine is selling portions of Xmas moss quite cheap too. You could also post a wanted thread in the Buy/Sell/Swap forum, I'm sure there's lots of mosses around on the forum that people will be able to send you free/cheap
I'd echo Steves comments. You will find that some mosses grow at different speeds. So if you are looking to cover a large area that is good to bear in mind. (Flame moss is noticeably slower for me) I have gone for little bits of different mosses all over the tank rather than a large cover of one particular type. I don't have any spare right now, but probably will in a couple of weeks. If you can't wait that long head over to the wants\swaps section
cheers for your replys.....i will head over..to the wanted section..wish me luck.. thanks for the advice given, i had an idea about java moss being the easiest to grow..its a nano tank ive got at the moment meaning..i dnt need loads. i now feel i should go with more than one type of moss to create a better lookin tank and experiment with moss to see what i like due to i have not actually seen any moss in person..only on the internet..lol