I have had mine from the dentist. I am not sure they were so easy to get when I started with mine, but I have seen them in the stores now. Mine is custom made, I am not sure how comfortable the others are. In 10 years or so, I have only had 2. When it gets worn down (which reminds me of the damage I am saving from my teeth), he re-lines it to strengthen it again.
I am a bad clencher. The grinding they call "Bruxism", which I do too. Even with that diagnosis, there is limited or no insurance coverage for these under most plans. It might be a couple hundred dollars, but for me, it is well worth it for comfort and for saving my teeth.
Let us know what you decide!