Toluene is 114 octane, available at any hardware store. Might be a good stop-gap measure! Mix it with the 87 and you'll get to 93 quite easily.
However, I haven't researched it for our cars, so search a bit more to ensure it's safe. (Though it worked perfecty fine on my Stage 2.5 WRX STi)
Grey Matter:2008 E92 M3:608/425. SG/PS SOLD!
Dark Matter:2015 F80 M3:495/505. AB/CB
? ? :2016 F82 M4 GTS --- Soon