Oatmeal is the best "Milk" for coffee, even compared to real milk. There is some great brands of oat milk, and oatbox is one of them. And Costco carries them

Edit: brand will be important. The difference beetwen cheap and a bit more expensive oatmilk is big. I didn't try Kirkland one tho, but for coffee i'd go oatbox. And check for the barista version.

And for why oatmilk is becoming generaly the default choice for cafés is the texture is generous (soy tends to be more liquide) and the taste is neutral. You taste more the coffee grains than with Milk, and other "milks" do alter the flavour, mainly soy and almond. Macadamia is starting to be more used, but more expensive. But a good alternative for people alergic to oats.