What is a website where I can find sheet music for Piano?
Not just the sheets, but with the music notes already on it.
It's not really a website but stevehedgermusic is his youtube channel and if you email him he'll send you his arrangements of popular songs and they are beautiful! I've played his songs for all of my recitals and have gotten a lot of complements.
Is there a piano version of UltimateGuitarTabs.com?
There are plenty, as long as you look in the right websites, or even ask around your friends - I find a lot of good quality music a lot in a lot of places. Just look it up in google :P