As far as I know there is no repository where you could download specific versions of Nintex Workflow. Also as a lot of bugs have been fixed since 3.1.8.0 I don't think someone would advice it to stick to that version. If your developers don't have a really good reason to stick to that version I strongly suggest you upgrade to the latest version (which supports all features of 3.1.8.0 anyway).

If you really can't go with anything else than 3.1.8.0 I can give you a link to download it but I suggest you speak with your developers first.

Best regards

Enrico

If you need older Versions you have to contact support

Ref: Downloading Old Versions of Nintex Workflow And Forms For SharePoint - Nintex Knowledge Base