<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by nerd:
Note of caution: according to this thread, the buses do not accept dollar bills, and there is no Metrocard vending machine at LGA. You can only get the free bus -> subway transfer if you use a Metrocard.</font>
You can buy a Metrocard at a newstand in the baggage claim area.