^ He's right, they can't sell ammo for pistols. .45, 9mm, .357, .38... they're not inherently pistol calibers just because the majority of guns chambered for them are. I always got ammo when I just said it was for a rifle.
But anyway, I don't suggest 9mm or .45 or anything for a truck gun, with so many other options out there.
You could get an 870, an SKS, one of the many Mosin Nagant carbines (M38, M39, M44), a Savage Axis in your favorite caliber, or a Rossi .357/.38 lever action all seem like excellent choices. The Rossi would be a little over $300, more like $400, but would serve you well.
Personally, I would put an SKS back there and call it a day. Reliable, cheap ammo, a fighter if you need it, a hunter if you need it, and can be picked up for $350 at the gun shows around here. In fact I plan on getting an SKS specifically for my Jeep once I finish this last semester, so I don't have to bother taking it out/putting it back every couple days (high five for being in nearly the same position... I turned 21 this year).