Silly question perhaps.

Where to find balloons with wide mouths to use receivers in?

Toys shops.
BTW receivers into a ballon will produce condensation due to dissipation, possible contact damage/corrosion is not excluded

Then what is your recommendation

I’d say use them - but cut open and remove at end of the day (or during lunch break) to allow any condensation to escape.

I know this will come across as strange, but a new rx each day when sailing is salt water. Since they are only $6 now I don’t mind tossing them when they get questionable. Granted I don’t sail every weekend is saltwater. I do have to drive 180 miles round trip to sail so I spend more on fuel than rx’s.

Cut the neck off the balloon or use condoms.

I have been racing F/E boats for 20 years. Its a 2 step process, #1 SEAL THE BOAT #2 Silicon Conformal coating, 3 coats on both sides of receiver (remove from the case). I also do the same to the servo board then seal the case seams and wire exit with a coat of Goop. Pull the servo horn off and coat the splines with a water proof grease and push the servo horn back on and you have created a waterproof seal there.

I have had boats full of water (broken water cooling line) for 1/2 hour and just dumped the water out sprayed the motor with WD-40 and was racing 1 hour later.