I bought these over the counter (avail at most US drugstores, and on Amazon); work very well and very comfortable, and no huge fee for "custom made" by dentist.
I Wish I would have known about these years ago:
Plackers "Grind no more" mouth guards
They come in a package of 14 of them stating "disposable" but instead of disposing of them, you can wash with castile soap, and with disinfecting mouthwash, to reuse them over and over.
(the same way you would wash the expensive type made by dentists, those expensive ones are made of plastic just like these reasonably priced ones are)
There are other brands too, but this one has a unique shape that I thought, from the photos, would fit anyone's mouth (and isn't bulky), and so I took a chance and bought them. They do the job for me and they are nicely comfortable for sleep in my mouth.