Just to clarify, you are asking if instead of the screen rotating based on how you hold it, you want to instead have a button you push to toggle the screen between portrait and landscape? If so, I haven't see any.
What I have is an HTC Desire Z. I have a widget that turns on or off auto-rotate. When I have it off, it is in portrait mode. If I open the keyboard, it switches to landscape mode. I close the keyboard to go back to portrait. I can do this since my phone has a physical slide out keyboard.