I’d suggest Landtop too. I went with a friend to buy hers from there when it first came out. They had some free 3-month insurance deal through Fubon that wasn’t available to foreigners until I complained about it, but it looks like they’re not doing that anymore for the iPhone anyway.
Costco probably isn’t the best, although I remember there being a time just after it came out where Costco would have been slightly cheaper than what my friend paid (at least when including the 2%/4% Costco cashback for using the credit card and executive membership). Landtop looks definitely cheaper now.
You can just type “Landtop” into Google Maps. There look to be three stores in Kaohsiung.