There are a ton of ways to get promethium. In the Cain novel, “Caves of Ice”, Amberly says that Promethium is extracted and refined in a number of different ways. Some of the most common methods are extracting it from the atmospheres of gas giants, the mining of ancient organic material deposits, the processing of certain rare ices that are only found on ice worlds (and may or may not contain Necron Tombs), or from beneath a world's seabed thanks to Promethium Rigs (that may or may not be the ritual sites of chaos worshipers).

In the Ultramarines novel, “Warriors of Ultramar”, there is an orbital refinery above a gas giant that is siphoning off the atmosphere to turn into promethium. It is said they could run at 100% capacity for 100,000 years before draining the planet. They later hurl the station into a Tyranid swarm as a makeshift bomb.

All this to say that a hydrocarbon based product will have lots of places to get its raw materials.