If you're willing to pay a couple of bucks, the Theme Song is available for sale here: https://www.sheetmusicdirect.com/se/...1/Product.aspx
If you're looking for some tabs for guitar/bas, here's a small collection of available ones: http://www.911tabs.com/tabs/l/lazytown/
As you can see, not much sheet music is actually on the internet for the songs. I tried asking Máni about any of the original scores, but they simply doesn't exist!
If you're looking for something specific, maybe I can help you out though. Our beloved friend Stingy has taken the time to create MIDI files for some of the more popular songs out there. These should be easy enough to transform into sheet music, and I actually did this myself when trying to learn Step by Step on the piano.
There's a dedicated thread to the MIDI files, but we have a few more unfinished ones than what's posted.