I bought some cute ones last year at Target online (long sleeve styles), but Sears/LL Bean and REI had some too. I waited for a sale and bought them for us (Sears/Lands End) and the kids (Target). Very happy with all of them
Instead of a cover-up, we always brought an oversized T shirt (Disney themed). Cover-ups are often thick and don't dry between uses. They also take up a lot of packing space.