If you want quality, you're better off going freshly roasted. That means buying it within a day or two of being roasted and using it up within 10-14 days at the latest. Nothing from Costco would meet that criteria--nor Starbucks, for that matter. Try to find some place that will give you at least some information about the region it came from, not just the country. Some places will even tell you which farm it came from and what processing method was used [ie. natural (sometimes called dry-processed), pulped-natural (sometimes called semi-washed or honey process), or washed (sometimes called wet-processed)].
I don't have any specific recommendations for places in the area you mentioned, but you could always try ordering from
Cafe Lulu in Taichung. Lulu is the barista, and her husband does the roasting. She speaks very good English, and she has a number of customers from Taipei who get her to send roasted coffee to them, partly because they like her coffee and partly because it's cheaper to run a business in Taichung. She'll grind it for you first if you want, but you should buy your own grinder if you want your coffee to taste better.