They usually have the packaged snorkel gear but Costco can suddenly be out of things.
Yes, more than likely they have it. Have you snorkeled before? I don't recommend buying a snorkel mask that you can't fit to your face. You can't do that with packaged equipment. A well fitting mask is essential to a good snorkeling experience. Nothing worse than a mask that leaks. Is there a reason you don't want to rent? If the snorkel mouthpiece is what bothers you you can buy a new mouthpiece, very reasonable and you can fit your mask at the rental shop.
I agree with Fishdiva, nothing ruins a snorkeling trip faster than a poorly fitted or poor quality mask. A leaking, fogging, mask is no fun. A rental gives you access to good quality equipment without having to buy it just for occasional use.
I’m with fishdiva andCarol H
Look at renting from someone like Snorkel Bob's. You can even purchase it if you like it. I usually buy from them since they can fit me. For better equipment try LeisurePro. Maybe buy equipment at home (snorkel and mask) then go to Walmart or something and just buy some fins to save space. We always travel with the mask and snorkel.
I'd also join the recommendation of either buying quality gear that you can try on first or just rent, and let them fit you properly.