Because I like pain and suffering, I recreate browser UI and form elements for my mock-ups from various sources for almost every other project. (Protip: This is how inertia and laziness manifests in more work later on.)
Today, I decided to get my act together and downloaded the Browser Form Elements PSD from Teehan+Lax, the same guys who made the wildly popular iPhone GUI PSD file.
If you need more UI elements from a slightly wider range of browsers, try the Designers Toolbox’s Web Browser Elements. Also recommended, but somewhat unrelated, is the excellent Yahoo! Design Stencil Kit.