A: Sonic Research Turbo Tuner ST-300 stomp box is the best option. However you can also get an AP for free - The Precision Strobe Tuner app is available on the AP store and Google Play store
Q: WHY CANīT I USE A STANDARD GUITAR TUNER?
A: The open strings are not tuned in equal tempered intervals. You can use and adjustable chromatic tuner with these offsets: Chromatic tuning offsets: E= -2, F= 0, F#= -4, G= +4, G#= -4, A= 0, Bb= -4, B= -1, C= +2, C#=-4, D= +2, Eb= -4. Open string tuning offsets: 1st E4=-1, 2nd B3=-1, 3rd G3=+4, 4th D3=+2, 5th A2=0, 6th E2=-2. However using the Tuner app or ST-300 will be MUCH easier.