Buying a Shipping Box for Guitar Amp?

if you have a local U-Haul store, check there too.   Walmart also has pretty cheap boxes. They have a light and a heavy version. I'd double box with the cheaper ones for sure.

You can cut down a larger box and make it fit.  Just make sure you don't have too much loose packing around it (like don't just pack it in a bunch of styrofoam peanuts).  Loose packing allows it to move in the box and as soon as someone drops or throws that box, it's game over.  I will generally wrap a heavier item in a couple layers of bubble wrap and then make the box "fit" snugly around that.  Double box is even better.  if you use foam or something, just make sure that it's "snug" and not loose in the box.  If it's too tight then it won't absorb impact, but if it's too loose then it moves too much.   You could also tightly wrap a box around the item, then put it in a larger overpack with some peanuts or bubble wrap - as long as it's in there snug.  When you pick it up you shouldn't be able to feel it slide around.  (I've had too many things show up damaged because they were NOT properly packed). 

another option is take it to a place that knows how to do this - but you will pay more.