Looks like it takes a standard Tyco R/C Flexpack 7.2v battery. If so it's pretty common and you should be able to get it at your local hobby stores.
Tyco R/C's web site refers you to Mattel Consumer Relations for all their batteries. If you prefer to order online from them, you can take a look at the selection at https://store.mattel.com/CatalogBrowser.aspx?cmd=local&cat=19&sec=8241
Alternatively you could take the old battery pack to your local Batteries Plus. If they do not have the exact replacement item, they can generally rebuild your pack with new cells very reasonably. If you do not have a local Batteries Plus (unlikely), you can talk to them at 800.677.8278.