Spare parts shop lah, where else. try Sea Park. Anyway, mine was not cause by the hinge, my good mudguard friend fix mine foc. It seem some door that are heavy like fatty will tend to drop, and it can be adjust. So I am keeping the 4 hinge in my mini spare parts cupboard now.
To check, you can try move your car door up and down, it should not have any free play even tiny one. If it have, is time to change. if it don't have freeplay maybe your hinge have move, it just need to be readjust. but most mudguard will ask you to change since it take the same effort to change and adjust.