This is a standard lab for 1st semester organic chemistry.  Go to your local college and buy a copy of "Pava."

An ordinary teabag will yield between 0 and 10 mg of caffeine, depending on your skill.  I think the Pava book uses dichloromethane in one of the steps, and this should be changed to a greener solvent.

I suspect it would be just as easy in a home environment to put the tea into an evacuated home canning jar, and deposit the caffein on the lid using dry ice to cool the lid.  I have not, however, attempted this.