If you search in Health and Fitness or Sports forums you will find a few different places.
I always buy from this website http://www.pcstore.com.tw/takmg/
, but it's all in Chinese.
You can also try the store that's near ZhongXiao Dunhua... Power House I think it's called. It's on the third floor of the building that has the fried chicken shop in it... if that helps. It's not that cheap though unless they have a sale.
Costco has it. I don't know what it's like though, as I don't go to Costco.
GNC sells it, but it's hella expensive and shit.
There's a few. I'm sure other people have other suggestions.