I’m want to get really into eating fruit, and I would like to start buying fruit by the case cause I know it’ll be way cheaper. So where do all the local fruit markets/street vendors go to get their supply?
Thanks for any help! 
I’ve been there twice, bought fruit, but it was at the crack of dawn…a huge market of veggies and fruit, somewhere near Taipei main station. Maybe someone who knows chinese can google it. You have to be there before seven and IMO buying by the case, is not a good idea coz fruit goes bad really quickly in Taiwan and you have to eat only one variety of fruit coz you have so much of it. If you buy multiple cases, storing them is a big problem. So do what the locals do and buy a little at a time and oh it does not really work out cheaper - the transportation costs, getting up early and the rotten stuff. 
What some locals do is go in on cases together and split them; then they don’t have so much of one kind to eat, and they get more variety. Sorry, but I don’t have info on how to do it.
A lot of fruit vendors have their own suppliers which is why fruit can be so much better at one stall over the next.
I find it quite a bit cheaper to buy fruit across the river in Yonghe rather than in Taipei city. The fruit store I go to the most is on the curve of Zhongzheng Rd. near Yuxi St. It’s open 24 hours, has a huge selection (in season; nothing really interesting is in season now), way fresher or cheaper than anything near Guting or Gongguan, and they’ll cut up any of the larger fruits like watermelon, pineapple, or durian for you.