LeslieKay is correct about that the most reliable source for job information is from the MSCs themselves. Each company has a different protocol, I'm sure, on what gets posted on various job boards. However, it is very unlikely that any 3rd party job board will be as up-to-date and accurate than the MSCs internal board. There are also some jobs that are never posted on the boards. Schedulers will contact reliable and tested shoppers directly to set up the shops.
This is a new one that I forgot to mention: [
www.jobbolt.com]. It just started this year, so I'm not sure how many are using it.