Whilst our users may be able to help you out here, you might also find our recent six-part series "Finding and Understanding Sources" a useful resource.

As a general reminder to all who might contribute, please keep your answers focused on OP's specific request for guidance on resources, as per our rules on advising on university and school work.

Generally, the best way to find primary sources for a topic you're researching is to find secondary literature on the topic, and look through their bibliography and citations to find relevant primary sources. You can then work on figuring out how to get those primary sources (online archives, library archives, interlibrary loan, etc). /u/sowser has an excellent suggestion in checking out our series on sources.

Now for more specific details, do you have an area of the world that you are focusing on? Such as UK, US, Asia, etc?

For the UK, you might be able to get a lot of information about poisoning murder cases from the trial records of the Old Bailey in London which are digitized and available for online searching.

A book that may be of help to you is Emsley, J. (2005). The elements of murder. Oxford: Oxford University Press. which you can preview on Google Books.

"Murder Most Foul," by Karen Halttunen might be a good source. It's about the emergence of "true crime" accounts in the 19th c., and it may have some good sources of about poisoning/accounts, etc....never mind that it's just a damn good book.