You can get these all over the place. Fortess have them for around 100 - 120 dollars I think (4 x RCA input, one output), but if you look in the electronic shops in and around Admiralty, Wan Chai, or even the locally owned little hardware / electronic stores in Central (a few in Stanley Street) you can probably get a cheap one for about $60. Don't pay more than 150 for one, or you're being ripped off. $50 will get you an ugly but effective one and $100 will get you a more polished one that does exactly the same thing.