Any dentists or dental health professionals out there?

I grind my teeth when I sleep. I’ve been doing it for years; it’s not just a temporary response to a passing condition. I was advised to get a night guard. The problem is that it is very expensive to have them custom made. Even at the local university where the dentistry students make them it is over $300, which is out of my price range. One time a dental hygienist said that I could get one ready-to-wear from the store. I see these range from $20 to $100 dollars, which is more affordable. My questions are: (1) Will a ready-to-wear night guard function as well as a custom-made one? (2) Which brand, style, or both, of ready-to-wear night guard should I get.