The reference to usscouts is perhaps the best "site" for information. It's been updated with the new ones. Several new ones were introduced at National Meeting this year. I've already seen one Scouter wearing one (Boyce), except it was on backwards. And many knots are on backwards, in real life, and on various websites, haha. Stick with usscouts as some private sites only list a few of the knots, and then are never updated.
Yes there are some you'll qualify for if you do the tenure and training requirements. Others, as you read, are bestowed on you from other Scouters or other organizations that support Scouting. We always remind ourselves and others, to not seek the knot, haha. If you earn one, then you should feel proud to wear it. But don't ever recommend yourself as qualifying for a special recognition, as it disqualifies you. And remember the "knot" is like a Military ribbon in that it isn't the Award....it only represents it. And you don't have to wear them, but wear them per Insignia Guide...keeping the distance from your stars if worn also.
Now, if "web" or moderators permit it, I can tell you the best privately produced book on all the knots, is a guy in my State. I don't have his '05 booklet, but it depicts the new ones. It depicts all versions of each knot, so it's a nice history book too. It also contains images of all the devices for the knots as well. Yes, there are requirements listed also, but depend on the official ones on your Progress Record for that.
