Music Composers Unite!
Here's a good link for you: http://musiclibraryreport.com/
It lists A LOT of the online library companies out there at the moment but also gives composers the chance to leave reviews. Spread this link around because if companies are getting a bad reputation from the composers who use them then it can only help weed out all the rubbish ones :)
If anyone else has had a bad experience with a library company then this is the place to get it off your chest! http://musiclibraryreport.com/
I'm hoping this will spread throughout the composing community as a standard 'checklist' before getting involved.