Dribbble is a community of the best graphic designers on the internet. They only take the best of the best and they require an invitation from a respectable member of the community if you want to join. Majority of the stuff the designers on dribbble create are put online for free download. There’s a big list of free downloads at the link.
Note: PSD is the Photoshop file format, so you need (at a minimum) Photoshop elements to modify the files.