Party City does not offer helium tank rentals, as of June 2015. They do sell helium tanks, ranging in size from 8.9 cubic feet to 14.9 cubic feet, with and without latex balloons.
Party City's website offers an 8.9-cubic-foot helium tank for $49.99. The same tank is available with 30 latex balloons for $54.99. The larger 14.9-cubic-foot tank is $59.99 and does not include balloons. The two tanks without balloons are available in-store or via ground shipping to the continental United States. The smaller tank with balloons is available online only.
The 8.9-cubic-foot tank holds enough helium to fill 30 9-inch latex balloons or 16 18-inch foil balloons. The larger tank fills 50 9-inch latex balloons or 27 18-inch foil balloons. The included instructions provide information regarding tank disposal. Tanks are not refillable and should stay out of the reach of children.