If you are looking for a diary for a specific person or family name, contact local museums where they would have lived. Through a wider net by putting your search on eBay and then making it a saved search. They send you an email when anything related shows up for sale.
If you've taken the Ancestry DNA test, you can contact all the 4th and 6th cousins that show up through that and ask if they know of any family diaries.
If just looking for 19th-century diaries in general, the eBay search is the way to go. There are many that can be read online from various libraries if you just want to read some and not necessarily own one.