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There is a good article in the archives by Chris White of White labs .
This article details everthing you need to know about culturing yeast lees from commercial beers.
Coopers is a nice easy one to culture.
I cutured some from the strong ale they make.
Other beers can be very difficult without plating out and growing individual colonies etc.
My opinion is, buy a known liquid yeast that will probably perform better than a old bottle yeast anyway (more viable cells free from mutations etc). Then keep the strain going. (storing and reusing)
If you want to experiment then mix some of the strains you have (a common belgian tech).
SiMo
