Electrocrytic
100+ Head-Fier
Original one is a 4000mah li-po battery. If you search in Ali express (for example) for your model you will find multiple ones. I've bought one from a trusted seller with 4500mah which should be suitable for my M6 Pro model.
Hopefully it's not a HSABAT or LOSONCOER 4500mah as they always have fake mah on the fancy label. If its one of those it will be more like 3000mah. Sometimes you don't need a battery analyser to check it, peeling off the fancy label can sometimes show the real capacity if they didn't fully alcohol wipe off the printing or have just put a black marker line over it.