Adding a Lean-to on the side of a building. Anyone know of a good kit?

Not sure you're going to find a kit. Typically, a lean to is designed into the structure of a building (pole barn?) from the beginning since it will affect the structural requirements.

Living in VA, you're going to have a snow load so there is that to consider. Not sure what steel prices are like where you are, but in S.TX, I can get steel pretty cheap and lot's of carport etc are built using 3 or 4 inch square tube, 12 or 14 g with steel trusses. I'm personally not a fan of versa tube, but that's just me.

One the the versa tube buildings have going for them over an open structure is the skin helps keep it rigid. A lean-to would be like a kite in my mind.

Good luck