it seems really exspensive on ebay for a little 5cm by 5 cm bit, is the price about right and if i got a bit does it grow and spread quite quickly? My link
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As Goodygumdrops said, you can get java moss from the classified section on this website.
Quite a lot of people will sell them as "golf ball sized" which doesn't sound a lot, but does go quite far considering you only have to put a few strands of it onto the wood or rock and it will spread.
Also, if you have too much then you can just put it in a bucket and it will grow fine