What is the best way to get current, up-to-date and historic
television ratings info, particularly for sports events (NBA, NFL,
MLB, NHL)?

- How are ratings compared (local vs. national, cable v. network)? 
- How does a ratings point equate to a number of viewers?

(I need a credible, sustaining source that can be referenced and
re-referenced for use in an academic paper)

Thanks!

For something credible, your best bet is going to be with the actual
Nielsen Ratings.  Some reports can be found for free, though a lof of
the detailed, market specific reports are for purchase only.  As it's
for an academic paper, I wouldn't advise getting a precanned summary
from some unknown source, as you won't know it's original source
material and the composition methods used.  The Neilson reports can be
found at

http://store.vnuemedia.com/nielsenmediaresearch/store/index.jsp

Hope this helps