I don’t know why you are “positive” any possible other food you could get would not be as good for her.
She may have just decided she didn’t like the taste of the Costco food any more, but it wasn’t really harming her in any way. If I had a big bowl of broccoli every day for breakfast, it would probably be very good for me, but it won’t be too long before I can’t stand the stuff any more.
Heck, there are plenty of dogs that live long and happy lives on the cheapest slop on the shelf at Wally World. I’m not advocating you go that low, just saying that there are probably plenty of cheaper options just as healthy for your dog.