It sounds like you are asking for a solution like this - that works for forums.
http://affiliate-blogs.5staraffiliat...e-links-blogs/
The script above it free but only works with Wordpress blogs.
Something that will auto-link certain keywords or phrases with your affiliate link? Well it depends on your forum software. I was looking for the same thing a few months ago. I found a script a few months back that does it with Vbulletin but it didn't support the version of Vbulletin I have. If you have Vbull search on the Vbull forum for things like autolink or auto-linker and you should be able to find it. If you have other forum software, go to their support forum and search for the same or variations.
If you can't find anything, post the forum software you are using here and I will try to look for you if I have time.
Hope this helps and best of luck!