Where can you get another dig from if you have already used from the fossil..

Raise a Trapinch to leval 41 and don't let it evolve at leval 40 because it learns dig at leval 41.

There's no way to get another, but trapinch learns it at level 40 or 41.

No you can't.

But you could catch a pokemon that automatically knows how to learn dig.

I heard Trapinch learns it at like level 43 or something IF you DON'T evolve it.

You can get another dig by:

Catching a Chemico(found at mount pyre) and training it to a high level I can't remember or find a Trapinch and train it(without letting it evolve).

The reason you have to be able to get dig and unable to delete it is because you need it to unlock the regis.

Dig is learned by trapinch (lv. 41) and nincada (lv. 45). I suggest that you go with nincada, since it doesn't need as much exp. to gain a level as trapinch, learns a few useful moves such as false swipe, and keeps you from getting a flygon which is stuck with dig instead of earthquake (it will only learn one of these attacks, and which one it learns is determined by the level the pokemon is at when it learns said move).