For the US, between $125 each (for 1) and $1.50 each (for 1000) from the official source, a company called Bowker. The structure is described in Bowker’s FAQ [0].
In the middle is 10 for $295, which is $30 per. That's a small enough number that you can split the cost with a few friends. But you can also get one from Australia, $88 for 10 or $44 for one.
