If what you are looking for is a cello solo played by someone like Yo Yo Ma, then I highly doubt anybody will have sheet music for it. That music might my copyrighted and therefore not for sale ever. You called it "cello song", I hope you know thats extremely vague, assuming that's not the actual name of it.
Notice how all the sheet music for sale is from Conductors that are LONG dead! Although I've seen easy sheet music from Disney cartoon movies.