A 10-lb. turkey is around NT$1,000 at Wellman's. I'm going to price Costco's turkeys later this week.
I'm not sure if you can get French Fried Onions anywhere else...so if you're determined to have green bean casserole, you'll have to buy them here.
Stove Top Stuffing is definitely cheaper at Wellman's. You can get two boxes of it for around NT$200. One box at Jason's is NT$150.
They also sell a frozen pie crust you can use if you don't want to make your own crust, as well as frozen tubs of whipped cream. But both are expensive, so I recommend attempting a homemade crust and buying whipping cream at Carrefour or Costco.
I think Wellman's also sells Cranberry sauce, although I forgot to look for it today.
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If you want evaporated or sweetened condensed milk, visit the EEC Filipino store on Jhongshan Road. It's quite a bit cheaper there.
And you can get chicken broth for NT$29 at Wellcome.
For rolls, try Costco, if you don't mind buying them in bulk.
And, of course, there are the mashed potatoes...visit your local vegetable vendor for the potatoes. If you want sour cream, the best deal is at Costco. You'll have to buy a big tub of it, but it will last you awhile!
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More ideas to come...