Almost every estate agent in the country uses RightMove, Zoopla and/or OnTheMarket nowadays. (Ask yourself if you'd even consider selling a house through an agency that didn't use one of those listing services. I suspect that, like almost everyone else, including me, your answer would be 'No', simply because everyone wants maximum exposure for their property).
So, once you've looked at those three sites, you've almost exhausted the entire market. There are still a few niche listing sites though that you might want to consider (although they're generally for mega-expensive properties)
https://www.theguardian.com/money/2013/sep/24/property-websites-alternatives-rightmove
You can find the websites of estate agents in Exeter
https://tinyurl.com/fchnty4y
and Plymouth
https://tinyurl.com/2kyh4uyf
via Yell.com but I doubt whether that will add much to your search.
The days when, for example, newspaper boys struggled with their loads on Saturday mornings in Sheffield (where I believe that you're currently based), due to the weight of the property supplement in the Sheffield Telegraph (or the Morning Telegraph, as it then was) are long since gone, due to the coming of the internet. Many local newspapers no longer exist in print format at all, with only online editions surviving. Even those that are online either have no property ads or alternatively provide only a feed into a national advertising service. That's how Devon Live (based in Plymouth) works, with their property ads link actually going to the national MarketplaceLive website:
https://marketplacelive.co.uk/national/property
(Devon Live and Plymouth Live, which carry much of the same content, are actually the websites of the Plymouth Herald).