you can't buy it,but you can get it in fallarbor town in one of the houses with tunnels
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you can get 28 dig on route 114
Dig is a TM, so you can teach it to it at any level.
That TM is given by a kid in the fossil maniac's house which is west from fallarbor town.
The TM for Dig is given to the player by the Fossil Maniac's younger brother in Pokemon Sapphire. It can also be purchased at the Lilycove Department Store as TM28.
TM28 Dig in Pokemon R/S is on Route 114 inside the Fossil Maniac's house.