I have been using Avgas in my toys for 35 years, yes it does have lead in it, at least it used to, I have never had issues running the Avgas, for me it is just a cheap alternative to race gas to prevent detonation.
I don't tune my toys to the edge either, they don't foul plugs and they don't burn pistons, that is the sweet spot :) I don't worry too much about cool weather vs hot but I can tell when it is cool the
Engine is happier.
I buy at local airports, maybe call around to explain what you are using it for, Avgas doesn't have road tax so it would be illegal to run in your car, if explained that you are only using in an off road vehicle, I would think you could find someone to sell it to you, if all else fails, most of the airports around here have self serve for the aircraft that come in on off hours :)