Walmart offers the SwimWays Star Wars Inflatable BB-8 Pool Toy for $16.99. (Amazon charges the same.) Opt for in-store pickup to dodge the $5.99 shipping charge. That's the lowest price we could find by $3. It measures 28" x 28" x 36" when inflated.
Kind of a cool note, this is "water weighted" so that it remains upright in the water. I was thinking it would be pretty sad to have BB-8 fallen over in the pool, but the mechanism to keep him upright makes this more exciting.