Have you checked the RSPB Shop?
If not, have a look at the following link: https://shopping.rspb.org.uk/wildlife-friendly-garden/wildlife-garden-cameras-detectors/nest-box-cameras/rspb-nest-box-camera-system.html
Also, if you joined via an RSPB shop or visitor centre, but NOT online, the staff there may be able to advise.
Mike
Flickr: Peak Rambler