J.P.
Go to a swim shop, or call a place that offers swim lessons. Most prefer, and/or require the reusable diapers. And most will let you try them on to get the right fit too. These are the complete diaper, you don't need anything more than the swim diaper (no covers, etc.) Just be aware that they are not water resistant. In other words, if you put them in the swim diaper, and load them into the car, and they pee on the way....wet carseat. Yes, that is from experience. :) The point of them is just to keep the solids contained. I have also bought the ones at onestepahead, and at 5 months, I think that the diaper designed ones are easier to take on and off than the pull up ones.