Have you tried the British Museum catalogue? They have several areas you could try, including online collections and publications. Museums have varying degrees of what is available, but if they have a library you may be able to get scans sent to you or arrange to view texts.

Also try public libraries - not necessarily your local, but big ones, like NYPL or the British Library. Univerisity libraries have always been useful, although not much unless you have access - if you find a university that has a collection of papers, you can always try to request copies of things from one library to another.

When I'm looking for older texts that are likely out of copyright, I have a look with the Internet Archive to see if they have anything - I'm rarely disappointed, and I've successfully found obscure reports and odd things that I though no one with access to pointy cutlery and public transport would willingly choose to site down and digitise. Project Gutenburg has old texts that are out of copyright, although they are more literature based.

If you feel like contributing to digitising papers and records, there are sites out there like MicroPasts, Old Weather and Zooniverse.

I've always ended up emailing people in archival and research departments, in various institutions, asking for help. They may take a few days to get back to you, but worst case they can help point you in the right direction.