Sunoco still sells leaded racing fuel. It has high levels of lead, enough that if you put 5 gallons in a thankful of 91 E-0, it will boost the octane really nicely. It is comparable to what was available at the pump until the late '70s, when leaded premium went away. I have a couple dealers near me that sell five gallon pails of high octane leaded race gas. Check this site to see what is close to you.
Sunoco Race Fuels
Of course lead is a poison, and removing it from gas was a good thing. Also, using off road fuel in your car on the highway is illegal. But I'm not preaching.
