You can make your own disco light similar to a stroboscope light using a standard tube lamp (TL). A half broken lamp with broken heating filament on one side can even be used, like the following circuit, it uses only one heating filament inside the tube :
Parts list :
P1:10k; P2:10k; C1:0.1uF; C2:10uF/16V; C3:47uF/16V; TUN: 2N3904 or BC547; R1: 470k; R2:100k; R3:3k9; R4:18k; R5:10k; R6:2k7; R7:33k; R8:1k; R10:2k2; R11:2k2.
The audio transformer connected to your audio amplifier is the only insulation. Even though the control circuit uses 12volt supply, it is not isolated from the SCR driving the high voltage. Due to this restriction, you have to be aware that this circuit uses high voltage from your main power line.