As far as I know, there isn’t any such repository. We have discussed setting one up in the past, but unfortunately it’s always been one of those lovely ideas which is really hard to put into practice.
Many of the people who give really good PowerPoint presentations are very reluctant to share their slides in an editable format. This is understandable because of the risk that other people will then introduce content that the original author does not condone, and then redistribute the slides with the original author’s name appearing to be associated with some nonsense about supermoons.
The other big issue here is copyright. Basically, all outreach presentations will have images whose copyright is owned by the original observers. Your average local society will typically operate on the basis that nobody is going to complain about their images appearing in a talk – but as soon as you start distributing the slides online, you are redistributing people’s work and clearly need consent to do that. Within the BAA this is managed through our T&Cs, which say that any images submitted to sections or online can be reproduced in any BAA publications, but that does not extend to inviting local societies to edit and reuse the images and share them online.
Sorry if that all sounds very negative!