Ring Doorbell 2 mains powered

Hi, I'm looking to being able to plug in my Ring Doorbell 2 so I won't have to keep charging the battery. I've seen many third party plugin transformers but can't find a official Ring one? I've also read a resistor should be fitted but none of the third party transformers say you do, so I'm very confused? Any help would be great please. I'm based in the UK, in case it makes a difference. Thanks