I don't know on what site you made your research, but Sentinel Mouthguard isn't that good from my point of view. It didn't even made it in the top 5 on this page: www.cureyourbruxism.com/top-online-dental-labs-custom-night-guards/
That being said, a mouth guard is a must. My experience is get one as soon as possible. And don't hesitate to pay a little a little extra to get faster service and shipping since every night without your night guard is a night where your teeth get a little bit more worn out.