Low Breakover Voltage Diac
Most diacs have a breakdown in the 30V range. You could use SIDAC, which has a breakdown in the 7~9V range. 

If you don't need the bidirectional characteristic, a programmable UJT such as the 2N6028 could be used.

But for most circuits requiring a threshold, comparators, op-amps used as comparators, discrete transistors and zener diodes, or even microcontrollers with ADCs are probably better modern-day solutions.