I have a document with several hundred footnotes. I want to find a specific one, (# 267 for example). Neither Find nor Find & Replace can find the footnote number in the text. Find does a “Navigate By” setting that allows footnotes to be browsed one-by-one, but this does not find a specific footnote number. Is there a way to do this?
I think one can now simply use the Navigator (F5). Under Navigation select footnote
This works. I can’t find a way to select your comment as the answer, sorry.
I’ve found no way to do this directly in Writer.
A clumsy workaround exists, such as saving the file in .fodt format (“Flat XML ODF Text Document”). You can then open this file in a text editor (not Writer), figure out the footnote markup in the XML, and search via the text editor.
You can also do the same using the .html format (“HTML Document”), but again you need to open this file in a text editor (not Writer or a web browser), figure out the footnote markup in the HTML, and search via the text editor. The .fodt method seems a bit easier.
Some other file formats may also work similarly, but I have not tried them.
You can use the Alternative dialog Find & Replace for Writer (AltSearch) extension.
Let’s suppose you are looking for the footnote with number 15. Once installed, launch the tool, check the “Regular Expressions” box and on the search field type
[::Footnote::]15
This will search for any footnote anchor with the string “15” on it. Yes, it will also find the 151, but hey, nothing is perfect 
One of those times an “another answer has been posted” alert would have been helpful to me. 
The Alternate Search extension does this very easily. It’s under the Extended drop-down category. If you are looking for footnote anchor content, the string will look something like this: [::Footnote::]11. You can also search for strings in footnote content.