Besides TM 28 Dig that you get from the Pokemaniac in Falabar Town, there is no other way to receive another TM Dig without the use of some cheating system/program.
That being said, you stated that you want Dig for the Regis, but only Regirock out of the three can learn it at all - and unfortunately that's via TM Dig as well.
However, if you really want Dig, you can indeed catch a Nincada or Trapinch who learn Dig at level 45 and 41 respectively. Or, if you don't want to catch one and instead have other previous generation games, you can catch a Diglett from Diglett's Cave, migrate it to Pokemon Emerald, in which Diglett/Dugtrio can learn it at level 17.
Hope I helped! :)