I just replaced 3 8x7 doors with Clopay uninsulated doors from Home Depot. They were cheap ($224) and came pretty well packaged.
There are a lot of different options with doors - insulated with steel on the inside will be more durable and heavier. The thin stamped steel doors like I have are indeed, thin and lightweight. I plan on insulating them myself though. Somewhat flimsy, but perfect for what I wanted.