Training Zombies in Black Ops 2

Town is the easiest best place to train zombies. You are in an open area with multiple routes.

You can train a group in the area where the bus parks by running around the perimeter. This is the area between the bank and the bar. The bar also contains a pool table which you can use to turn and shoot at your train. You can exit up the stairs, then fall down into the area below (therefore allowing you to repeat the building of a train).

You should be able to rinse and repeat with some practise. You are next to the bank so have the option to drop off points for future games. Jug is nearby too. If you get into a jam you can run down into the mist or back into the main area around the corner.

Alternatively, you can avoid training zombies completely and just hold out under the bank. You have access to a pack-a-punch machine under the bank. Put 1-2 people at the top of the stairs facing the corridor that leads to the main bank area (don't go beyond the hole in the wall to the right). Have the rest of your group in the tunnels below making sure nothing gets out of the windows. The 1 or 2 guarding the vault entrance may rotate with people below to spread out the points being earned.

Update

Having watched several videos, the diner seems like another good place. There is an open area where the bus exits the area, between the mist and the slightly open garage door. You have plenty of room here to run circles. If you are working in a full group then have people split up (maybe 2 outside bank, 2 outside diner). It gets too overcrowded on later rounds for everyone in the same area.

As you have mentioned ammo being a problem, I would suggest you want to have 2 pack-a-punched weapons (something like starting pistol and ray gun for example) without relying on guns from the wall. If the bank vault is open, power is up and the box is in the town you can rotate guns to increase your ammo (replace empty weapons, then pack a punch the new gun). Other than that it is just a case of making the most of your weapons (explosive rounds into large groups of zombies, headshots etc) and picking up max ammo when it is flashing (emptying your gun whilst you wait, chucking grenades/emps too). Having the knife or knuckles can help but loses its effectiveness during later rounds. Its feasible to use knuckles only for the first 10 rounds at least, without ever needing to fire a bullet. The only other tip I can give you is to control the rounds (leave a couple of zombies at the end of each wave) so that you can get perks/new weapons without a horde following you.

With my experience on town, where jug is there is a good point weapon on the wall and I have seen many people train around there after buying it and you get bigger groups while training so points is not a problem and if you open the door to where the box can be, training is simple and on solo, you cant go to very high rounds without quick revive due to it reviving you if you get down on solo.Some You-tubers train before they get their pack-a-punched weapon.