Tetra-ethyl lead (T.E.L.) is still for sale in bulk,
and is of course used in Commercially sold aviation gasoline.
You must use protective gloves when blending in TEL as it is poisonous.

MMT is for sale, and is cheaper in bulk by far, than the small cans of MMT containing octane booster sold in auto parts stores. MMT is still widely used in overseas gasoline, and is advertised as a “green chemical” because it reduces greenhouse gases emitted per mile driven.

The first octane booster discovered was Iodine, like used as antiseptic for cuts.

E85 is an readily available alternative with higher octane 105 AKI alternative if you are willing to change air fuel ratio.

Methanol and water blends sprayed into intake manifolds both boost Octane rating, and cool the intake charge for an additional power gain.