I have a stove top espresso maker that isn't working properly anymore.
Pressure is escaping from the safety valve before enough pressure has
built up to force the water through the coffee.

Does anyone know if it's possible to get new safety valves for stove top
espresso makers? They easily unscrew would be easy to replace if it was
possible to get hold of one.

You certainly can. I've bought new gaskets, filter plates and escape
valves from a local Thompson coffee shop here in Glasgow.

Richard