What instruments are used to indicate aircraft attitude in acrobatic airplanes?
The basics:  Acro airplanes don’t have attitude indicators because they don’t need them.  The airplanes are certified at Day/Night VFR flight only and gyroscopic instruments are not required equipment here, not to mention can be precessed and damaged by aerobatic flight.

The only attitude instruments needed here is the horizon itself and the pilot’s Mk I eyeballs to see it.

Sometimes pilots make use of aerobatic sights mounted on the wingtips; these look something like eight spoke cowboy spurs or a 45° right triangle frame and are used to fly certain reference lines during aerobatic flight.

Some acro aircraft do make use of glass cockpits and solid state Attitiude, Heading, and Reference Systems (AHARS) as a safety backup on cross country flights, but they are not used during aerobatic routines.