Any good, substantial piano sheet music can't be found online for free. I've been spent countless hours searching, lol. You can find free one-paged samples tho...but really, not many people want that. Sheet music isn't that expensive (unless you're buying it in piano music anthologies and what not) if you want to buy it. I have some music at home that I used to play that is at that level...only they aren't labled "level 5" so I would have to know what type of music you're playing - also, I don't live at home so getting to the music would be delayed quite a bit haha. My suggestion to you is to take a trip over to a music store and search the music there. Some places have a great selection! If you don't want to travel, Amazon.com has some good music - just make sure you are able to take a peek at what the music looks like before buying. Don't rely soley on the level # - that is sometimes very subjective.
To play by ear you don't need to take lessons. You just need to know, basically, that the farther to the right you go, the higher the pitch gets and the farther to the left you go, the lower the pitch gets. If you know that, and you know the tune of the song you want to play, you just sort of play by what sounds right. It's easier for people who have perfect pitch (which is very hard to master). I, by all means, don't have perfect pitch and I've been playing the piano for over 14 years, almost 15.