Or maybe just ask the smokers to smoke outside??? I smoked years ago.  I had
3 roommates and out of 4 of us, 2 of us smoked.  Me and my roommate who also
smoked ALWAYS smoked outside, not only out of courtesy for our other 2
roommates, but because we didn't want our house to smell like an ashtray.

Smoking outside is a lot more enjoyable anyway.  It's not too much to ask.
If they say "no" given the problems the smoke causes you they certainly are
not your friends.

As for an air purifier, the best you can get is the Friedrich C90A,
according to Consumer Reports.  It's quiet compared to most HEPA's and it's
electrostatic so no expensive filter changes.  You just have to run it
through the dishwasher every few days which is no big deal.  I have one and
the air in my room stays amazingly clean.  Then again, I have hardwood floor
in my room which helps a LOT.

I got mine here:

http://www.genieac.com/c90.htm

Genie has raised the price considerably since I bought mine, probably
because of the popularity of the unit.  They still seem to be as low as the
other sites selling this unit.  The retail is $529 but without the added
cost of filter replacement this is made up for QUICKLY.