They sell little baggies at your LFS. If not, they have some at ***** or PetsMart I buy the bottled purigen and then purchase little bags there. Just make sure that the bag is closed and tied before putting it in the tank. The tiny beads of the purigen will go everywhere.
Also, before you put it in your tank, rinse the bag with purigen in it with some RO or salt water to make any super fine particles go down the sink and not float in your tank.