Outside of asking my pro shop, where else can I find (assuming the size) an 18x9x9 shipping box? I have a tournament coming up and I wanted to ship 2 balls.

Amazon will have them also. I got mine there

Just ask your pro shop for a case box and cut it down.  They throw them away every day.

I think Amazon getting into the food business also

They have for a while now: I think it's called Food Prime or something similar. They even sell wine and lobster.

I've ordered dry foods like tea and noodles and many spices, hot sauces and barbecue sauces that I can't get locally. Haven't yet ordered wet/fresh or frozen foods from them.

a 4 ball case box, a utility knife and a minute and a half and you have a 2 ball box.

Thank you for all of the links and home remedy solutions.  Fortunately my pro shop had a bunch of them, so I was able to get one from them.  If I strikeout with my pro shop next time, I will know where to go for boxes.