The shower drain pumps are just diaphragm pumps, it doesn't matter which manufacturer or model it is. You just need to match the size of the hose connectors to the hose you have. The flow rate should be the same or higher as the freshwater pump. Pressure rating doesn't really matter.
If you're smart, you buy the same exact pump as the one on the freshwater system. That way you can swap them in an emergency (after a clean), use the same spares and the shower sump gets pumped out at the same speed the water runs into it.
The triple diaphragm models tend to cost around €50-70 depending on the model.