How to find accommodation that is not listed in booking websites? [duplicate]

Search (on internet) on hotel and the name of the town, or use local names, like in England 'inn' and B&B might find you things you may miss when just looking on hotels.

If you know the city (town or village) you want to stay, you can look up the local phone listings. (Now online for most of the world.)

Look at the site of the tourist information office.
In many cases they list hotels in their area.

And if you still do not have anything, you can of course contact the tourist information office or in some cases to chamber of commerce and ask if they have a list which you have not found yet.

When traveling around, like by train or a road trip which is quite popular, talk with people who have been where you are going. I have found my best hostels that way, hotels are a bit less likely but still not impossible.

If you have a good travel agent, you may ask him/her if they know hotels where you are going.

You could try Wikivoyage. It is a travel guide that anyone can edit. Coverage can vary, some places are very detailed and up to date, others not so much. Depends on whether anyone has been there and added details to the articles. But there can be places not listed in any other official guides.

Of course, you can edit the Wikivoyage articles. So if you find a great, obscure hotel, you can add it. Along with a description to say why you liked it, as well as directions and coordinates so other people can find it.

One more thing which you can do is that you try to enlarge your place of interest on Google maps and see the various establishments coming up on the map.... and most of the time when you are doing that you can see many hotels and restaurants which are not in the booking websites. After you have seen them, then you can search by that name on Google for contact info.