I am working on a project that needs ambient street sounds from London in the 1830's. I have been trying to build this up from nothing and it is going well, however I need some sort of underlying tone to tie everything together. Does anyone have suggestions where to find something like this? The track is about 40 minutes long.
The two things that come immediately to mind are that you should look at photography and sketches of 19th century London, and you should listen to any audio material you can find from cities that are at about that level of technological development.
Photography and sketches will help you determine what noise-creating objects you need to simulate. It is safe to assume that there won't be cars and busses, but what sort of animals, trains, and steam engines should you expect?
One thing to keep in mind is that the British Empire reached around the world before it started to fall apart. As a result, you might be able to find cities and villages which were former parts of the British Empire that have retained 19th century British urban technology. Look for recordings of these areas, if you can find them. Or travel to them if your budget allows.
Finally, check out documentaries and period dramas set at the same time. If your goal is entertainment rather than documentary, then you may be better off sticking to the established cliches than authentic reproduction. There are some good articles about Foley effects that suggest that effective sound design is more important than authentic sound design.
Check out FreeSound to find sound design elements.