They should be sold at most hobby stores that sell trading cards, though it would be best to call ahead first.
They appear to be selling for $3.99 from this online retailer:
http://www.hasbrotoyshop.com/ProductsByBrand.htm?BR=855&ST=SO&PG=1&ST=PRDS
That link was provided by Wizards, the manufacturers of the Cards, so I would assume that's near standard pricing for a pack.