I don't know any good mystery adventures, sorry. But I have tried to run a few homegrown ones in my time, and let me give you some advice:
Be careful. These things tends to bring your game to a crawl. It sounds like a cool idea, but making a mystery the focus of your game can really make things dull. That's why if you read the submission guidelines for Dungeon magazine, they specifically say not to send one in. If you're a new DM, it's especially hard to run.
Don't let the mystery keep the game from moving. Put in lots of action.
Do find a published adventure, just like you're doing, rather than write one yourself. You're right to see what someone else has done.
One technique that you may or may not want to use is to create a mystery without a solution. Don't say to yourself "the butler did it". Instead describe the situation, and let the players run around as you continue to play NPCs off the cuff and describe their environment. They'll start collecting what they think are clues and will talk endlessly about who done it, in which room and with what weapon. (If they're interested in this sort of thing). They will usually come up with a more convoluted plot than you ever could. Steal their ideas and make them so. They'll think you're a genius and will be quite satisfied. Unless they catch on.... You have to decide whether you can pull this off or not, and how often to risk it...
In any case, if they look bored, throw some combat at them, quick.
