I've recently used 35% gel from a UK dentist and was really amazed at the results....I had two dead teeth at the front (from contact sports !) which were noticeably less white than the rest of my teeth..generally none of my teeth were very white, but using the gel for a week made such a difference. After trying the gel for 30 minutes, I use it at night in bed and have no problems. I've bought the mouth guards off the net, but they are not any good...for a good fit, go to your dentist and pay for a proper mould. The use product purchased from a reliable net source.
hope this is of some use.