Contact a local race shop, they can usually get you in touch. You may have to buy the 22 or 55 gallon size though. I had a hard time finding the 5 gallon sizes for my race bike. Or you can mix your own like I do sometimes... A little cetane, a little lead, voila! ~105 octane. Skip the lead if you have cats still!
Race gas The Shell gas station off Hwy 9 near crabapple should have 100 oct gas. Haven't been there in a while but last time I got some it was $5 / gallon. Have used it on both car and bike and it's good and not that expensive.