First of all make sure to have a recent unencrypted iTunes backup, like explained here.
Then you can use any iTunes extractor to obtain the required ChatStorage.sqlite, like the free ones provided below:
- iPhone Backup Extractor - free until 4 files extracted (enough to download the required ChatStorage.sqlite)
- iPhone Backup Browser - 100% free, open source, no limitations (could not work on some older PCs)
- iExplorer - free
- iBackup Viewer - free
ChatStorage is located at