I used www.Alansfactoryoutlet.com when I bought mine. They have multiple different style roof systems, sidewall and backwall options, different heights/lengths/widths etc. They also price them according to region.
In my area I had high wind and snow load issues to worry about, so it cost me more than it would if I lived elsewhere. Mine has standard rebar stakes and 6 additional hurricane ties as required for the wind load, plus additional vertical bracing as required for snow load. That was factored into the price, which was VERY reasonable for purchase, delivery and professional installation.
All I had to do was prep the area... make it level (within a couple inches) and cleared. I had to provide electricity within 100 or 150 feet (already there) and clear access to deliver materials. They arranged a delivery date, 3 guys showed up and installed it in a couple hours one morning. I was very happy with it then, even happier with it now.
Well worth the money to keep rain off the roof! We can pack/unpack in any weather conditions. We can keep windows open year round to prevent and moisture odors. Less chance of leaks, less damage from the sun. Its AWESOME!