There is no real (or linear) correlation between boost versus octane. Gasoline with greater octane rating is used in turbocharged (as well as supercharged) applications for the simple reason to help prevent knock. Because turbochargers make a bigger bang when ignition occurs, there is a higher risk of knock due to the higher pressures and higher heat created due to the boost. Your vehicle being old school, it cannot adjust for knock like newer cars can, therefore you need higher octane to help prevent it. It's not that it needs a certain octane, it's that it needs to be above a certain octane. As you suggested, that octane is 91 for your vehicle. If you ran higher octane fuel in it, it would be just fine.
Two things you should realize ...
First, most gasoline sold on the US market today has about 10% ethanol. This amount should be safe for even older vehicles. Running this in your vehicle should not give you any issues. It is recommended you do not go above this amount. Even 15% will start to degrade your vehicles rubber parts. As you've found out, finding fuel without ethanol is a chore these days. (Note: One place you can find it is at marinas ... I don't know if they'll have the octane you need, but they sell ethanol free fuel there ... mind you, you'll probably pay about twice as much as you're used to for it.)
If the above doesn't console you, the second thing you should start thinking about is upgrading your soft parts to the kind which can withstand even 85% ethanol, that way you won't have to worry about it any more.