So you want to make a Thanksgiving dinner? Well, there's no such thing as "one-stop shopping" here. You need to visit several stores to get everything you need. Here's the official "Food for Foreigners" guide to Thanksgiving shopping.
Costco (best deal, I think)
TAS (you need to know a TAS parent)
American Club (not a good deal...waaaay too expensive)
(as of 11/17, Wellman's didn't have any stuffing)
FROZEN GREEN BEANS
FRENCH FRIED ONIONS
can't find them anywhere. But you can try the Chinese version of fried onions...I think City Super carries them
you can find this anywhere...even at Wellcome
Costco sells them for about NT$300 each and they're big.
If you want to make your own, you can get frozen pie crusts and pumpkin pie mix at Wellman's.
you can get big bags of pecans at Costco.
The dark or light corn syrup can be purchased at Jason's Marketplace or Wellman's.
Also, the best brown sugar can be bought at Wellman's. Don't bake with the Chinese brown sugar. It's not the same.
MASHED POTATOES AND SWEET POTATOES
visit your local vegetable vendors. Marshmallows for sweet potato casserole can be found at Wellman's. Sometimes Carrefour carries marshmallows, but they're not the Jet-Puffed ones, and they don't melt as well.
Jason's Marketplace, Carrefour
Am I missing anything? Of course, this list isn't comprehensive...but hopefully it will give you a good start with your holiday cooking!