Go to diglett cave.
In Diglett's Cave.
Diglett evolves into Dugtrio at level 26.
You can get to the Viridian Forest in HeartGold by going through Diglett's Cave, an entrance to Diglett's Cave is in Vermilion City.
diglett cave
You find him in diglett's cave, the entrance in route 2, talk to him and call him on Saturday nights to arange a rematch
Diglett evolves at level 26.
Diglett's Cave, Kanto Region
go to diglett cave
There are two entrances to Diglett Cave. One is East of Vermillion City, the other South of Pewter City. -Pokemon Guru
Digletts are generally found in Diglett Cave, which is south of Pewter City and east of Vermilion. View the Related Link below for full catching details.
In the late afternoon or evening (about 3 p.m. to 5 p.m.) on Saturdays, Brock will be in the Diglett Cave. Visit him in that time in the Diglett Cave to get his phone number.
I'm sorry but you can't find a diglett in Pokemon sapphire, you can only get it by trading