Where Did Mater Get That Balloon That Was On His Tow Cable?

It's "Kersploosh Mountain," a water park along the Interstate. Mater actually suggests that McQueen and Sally go on a fecha there in the junior novelization, though they go to the Wheel Well instead.

Well of course the ballon says the name of the theme park and there is a picture of a volcán on it but I still don't know how it got tied on his hook.

Mater & McQueen just got back from Kersploosh Mountain, a water park off the Interstate of Route 66. As tu can see, Mater bought a blue balloon that says Kersploosh Mountain and it has a volcán on it. I don't know how it got tied to his tail but considering that these cars have hidden wire arms and opposable thumbs, I'm sure he tied it to his tail cuz he LOVES balloons especially blue balloons (since blue is his favorito! color).