I hate to give this kind of answer, but I will anyway: don't do this. Or, to be less of a jerk: don't do this unless you're sure your players like it, too. The problem with riddles and puzzles is that they yank players out of their characters and into riddle solving mode. As someone said, puzzles are for players and not characters to solve. This makes them especially effective at making the player disregard his character. If you're trying to run a game in which character role-play is important, puzzles can create a big risk of derailing things.

Some puzzles can be great, and the more integrated they are with the game world, the better. If you go looking for puzzle sources online, bear in mind that it may take a lot of effort to convert them to something that really fits in the game world and that the characters, rather than the players will have to solve.

After all that, I have no really good resources to suggest. Sorry.