Do you stock bathroom faucets...?

No

While some Costco's may stock hardware items, the Louisville/Superior Costco does not. With this store, you want to imagine mostly a grocery store, but bigger and on steroids. Though i am not sure about any of the other surrounding metro Costco's, you would have to call and check what sort of supplies each carried..

You have to call them

Not usually.

I have seen them there, I would call first

I've seen them stock some faucets from time to time. Maybe a seasonal item. I know you can get garbage disposals, too.