Not sure if you want to make your own yoghurt. I can tell you at the next playgroup if you wish. Most of the yoghurt sold here tend to have sugar added to it but I think there might be some foreign brands that aren't but pricey.
As for milk, we get ours from a bakery which orders from National Taiwan University (they've got their own cow farm), 四方 (Four Ways) http://www.fourways.com.tw/01/about.htm and 初鹿 (Chu Lu) http://www.chuluranch.com/.
From what I've found out, Four Ways and Chu Lu tend to be more organic but apparently there aren't any 100% organic milk made in Taiwan due to importing of grains and maybe hay. It would be too expensive to import organic grains. Chu Lu costs around NT$88 now and Four Ways NT$10 less for a litre. Hope that this helps. :D
Karen