Drilbur and Excadrill aren't native to any Alola game, so getting one there requires bringing it in from elsewhere. Those games can't trade directly with the earlier games in which they do appear, and what you have to do instead is move it from the older game into Pokemon Bank, and withdraw it from there into Sun or Moon.
If somebody else already made that transfer (as many have clearly done), another option is to have them take it to the day care to get more eggs of it, then hand those out over GTS or Wonder Trade. You could give that a try if you're genuinely looking for one.
