The chemist has ear plugs and also the ear silicone that you mould into the ear.
If it's for after the grommets Olive...I didn't with DS2. My ENT guy was pretty relaxed by it all and said that for the couple of days just don't immerse them in water (ie: pool) and be mindful in the bath.
You may find a swimming cap would do a good job if you put it over the ears??