You can from unfiltered beer. Here is a thread on some beers with yeast in them. http://www.homebrewtalk.com/f163/com...t-list-262713/
It's easier to take a specific beer and then search in google for something like "harvesting yeast from ________". See if anything comes up and what yeast it is.
I have a link on harvesting from beer in my sig, harvesting conan yeast, it applies to all beers.