What kind of flowers do you want in your bouquet?
I'm not sure about the Costco in Mexico, but I know for a fact that here in Montreal, the quality of the flowers is not always good. My sister has on several occasions made bouquets, and a few times when she ordered them from Costco, half of them were not usable.... just something to think of.
But you are there for a few days before, so you could always check out local flower shops and then assemble them yourself. Practice first!