The $7 Fun Pass is a MetroCard, so if Hudson News sells them they'll probably also sell $10 and $20 value cards---a better deal unless you'll be taking 4 or more subway/bus rides per day during your stay in the city.
Good to know that they're available at LGA, as not only is it hard to get $2.00 change for the bus, the MetroCard gives you a free transfer to the subway.