How can i get her out of this?

When i go out with my little girl, shes 2 and a half, and when she doesnt get what she wants she will go in a strop and just lye on the floor no matter where we are. the other day, she didnt get this ball thing she wanted so then would start crying and lyn on the shop floor, so i had to carry her all the way home screaming and wriggling in my arms. everyone kept staring, was so embarssing as u see other kids out who are well behaved.