This might be a dumb question, but where the hell do you get a box long enough to put a rifle in to ship it?

take a big box long enough, cut it down so it's not too big around and tape the shit out of it.  

Or just go to a gun store and ask if they have any shipping boxes.  

I second the notion of trying to locate a box at a local gunshop, as I've done it before myself.

Now if you don't mind spending a little money and have the time, you can do what I did to ship my M16 out of state for a refinish. I ordered a cheap hardcase from Bushmaster for something like $15 and it came packaged in a cardboard box. UPS charges more to ship an unwrapped hardcase, so the box came in handy. I then shipped the rifle in the hardcase which was inside the cardboard box that it came in. That way, I had no fear of any damage to my rifle while in transit.

Madman has it dead on!  Shipping in an inexpensive rifle case is the way to go if you do not want your rifle to get mashed by UPS!  I have had a couple rifles damaged by UPS when shipped in cardboard boxes wrapped in newpaper and styrofoam peanuts.

One had a broken wood stock right at the pistol grip, another a nice large ding on the muzzle of the barrel.  I am not talking inexpensive rifles, a Walther smallbore target and a match conditioned M1 Garand with Douglas heavy barrel.  One rifle I shipped in a cheap Docosil case survived without any damage though the end of the case was cracked and there was a hole in the side of the case I could put two fingers through.  This one was a double lugged M1A super match!  I now ship in a more expensive case, $50 type just to be sure!