The last 7 items on
the octane list (not including hydrogen) are safe bets, but how much of it you need to raise the octane and keep the A/F ratios in check are missing factors. Also finding them pure is hard. Toluene seems to be the easiest/cheapest and is the main ingredient in most octane boosters I believe (along with xylene). If you had 5 gallons of 93 octane (US) gasoline in your tank and put 5 gallons of toluene (that's a whole lot of toluene!) in I think you'd end up with 98 octane... But I could be wrong...
Isopropanol (isopropyl alcohol / rubbing alcohol) is easy to find (drug stores) in 90%+ purity but isn't terribly cheap. It would take a little less of it to raise the octane but still I think it's a bit nutty of an idea.
Methanol would be nice to use but I don't know how to get a hold of it easily other than mixed 50/50 with water (windshield washer fluid).