Online collections and bookstores are the best places to find challenging riddles. Good Riddles Now, Riddlers and Brain Den maintain online collections of riddles with answers. Most major bookstores, such as Barnes & Noble, have collections devoted to riddles, brainteasers and word puzzles for all ages.
"World's Greatest Riddles and Brain Teasers," by Dr. Edward Scarzi, is a popular book of riddles that also provides humorous content and riddles that are fun for groups. Gary Gruber's collection, "World's 200 Hardest Brain Teasers: Mind-Boggling Puzzles, Problems, and Curious Questions to Sharpen Your Brain," is a challenge even to experienced puzzle solvers. Both of these books are highly rated according to Barnes & Noble, which has an extensive selection of puzzle books.
Riddle collections available online provide a variety of tools that make searching for riddles easy. Riddle websites categorize riddles according to subject, origin and even tone, distinguishing serious and silly riddles from sexual content and separating out riddles designed for children. Rating systems allow readers to find the "best" riddles, which are typically the most challenging or unique, as determined by the websites' users. Websites such as Good Riddles Now allow users to create content, resulting in a constant stream of fresh challenges for riddle lovers.