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.