If you can read music, perhaps you want to take a look here:
Sheet Music Downloads & Books | Musicnotes.com
There's very few songs they don't have and some of them come in different formats. Some of them, you can even change the key it's written in. They sell the sheet music, but you can view the first page of the music for free, but if you want the entire song you have to buy it. The reason I'm saying this, is a lot of the time you can get 90% of the melody line by looking at the first page (which is free). This might speed up working on the melody. Just my 2 cents worth! Good luck.
hb