You know something the first time I have ever seen it in person was a few weeks ago while I was in Charlotte NC. I was at a Citgo gas station and on the pumps over on the left side (all by itself) is the 100 octane fuel. Thats the first time I have ever seen it, and havent seen it since.
Most, if not all, dragstrips sell it and even higher octane. Also, you cannot pump it directly into your vehicle as the nozzle is bigger than what fits in your filler neck. You'll need a gas can to put it in.
Ya seems like random gas stations. I know of a couple around here, one is at this gas station that is more for truckers then anyone cause its kinda in the country and the other is at this random gas station in the city. Just look around might get lucky and find one.