Where To Get High Octane Gas...

Used to work for an aircraft outfit. Most of the FBOs on our field would sell us gas in our gas cans. We would do this when we suspected contaminated fuel. (Fuel usually contaminated by condensation that occured when the wing tanks would heat and cool daily when moored on the ramp.)
The hottest AV gas these days is about 135 research and only required by high performance WWII vintage aircraft. If you're wired in with a warbird, this would be great stuff for a racing engine. I'd be very surprised to find it at a field with no vintage aircraft based there.
Most recips use 100LL (low lead) which can be used in a car. Price on this stuff here at the Edge of the World goes from $4.58 to $7.83 a gallon. The lower price is at the smaller airports. The high priced stuff is at the fancy schmancy FBOs on big fields. It's the same Shell or BP at both places.
Because no road tax is on AVGAS, they are prohibit to fuel a car with it. Since an aircraft owner can fuel his own plane, there's nothing to keep you from walking off with a bunch of fuel cans full of this stuff. The seller may require you furnish a "tail number". This may not be the most kosher way to get high octane, but it is a source.