I can understand the convenience and price of Costco (I'm a member), but their selection isn't the best. Also note that they will only install the OE tire size on the car.
Have you considered using
Tire Rack? They're my #1 choice, with a great selection and very good prices (even when shipping is factored in). The only time I don't buy from them is when they don't sell the tires I'm looking for (i.e. the Falken Azenis RT-615s I use for autocrossing).