Mine came from here - http://www.bikecare.co.uk/product_info.php?acc=Brake_Parts_Page_5&id=89
A bit of googling turned up a few other results. Search for "dual pull brake lever" or "double barrel brake lever" and see what you can find.
The absolute cheapest way is to make your own. I'm sure anyone who has made one will be happy to show you how.
@diceface started
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Mine came from here - http://www.bikecare.co.uk/product_info.php?acc=Brake_Parts_Page_5&id=89
A bit of googling turned up a few other results. Search for "dual pull brake lever" or "double barrel brake lever" and see what you can find.
The absolute cheapest way is to make your own. I'm sure anyone who has made one will be happy to show you how.