The OPC libraries are available through the OPC foundation website:
I believe that you need to be a member, but it was a fairly straight forward process.
Best of luck!
(edit> Had a bad link)
@Christopher: can I kindly ask what it is you need the OPC libraries for? If you are developing a new application that accesses PI System data, I would recommend you take a look at the different data access alternatives (i.e. SQL-based products, web services, SDKs). The PI Programming 101 webinar is good at introducing the various technologies but unfortunately it doesn't provide the information to help select the right technology in the first place.
The PI Data Access family includes various products and they all have their strengths and weaknesses. As it stands now, the PI OPC DA/HDA Server (which exposes the PI System as an OPC data source) does not expose as rich a feature set as other products do; for instance it doesn't expose the PI Asset Framework - AF - or PI Event Frames. Additionally, it may not offer the same levels of performance depending on the use cases.
So what is the use case? What are your needs and requirements?