You can find Makuhita in the following locations: Slowpoke Well, Union Cave, Mt. Mortar, Dark Cave, Whirl Islands, Mt. Silver, Ice Path, Cliff Cave, Diglett's Cave, Mt. Moon, Cerulean Cave, Rock Tunnel, Seafoam Islands, Victory Roads and Tohjo Falls however you need to use the PokéGear's Hoenn Sound channel in order to find it in those locations.
You check the radio on the poke-gear for the Hoen music. This only happens on a certain day, once the music is on keep it on and enter any cave in kanto. Walk around the cave and you'll find Makuhita.
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you don't
You can't, trade with soulsilver
to find Blue in HeartGold or SoulSilver go to Cinabar Island next to Seafoam Island and go right next to the Pokemon center.
You can only find Skarmory in the Pokemon SoulSilver version.
You'll need to trade for it, because evolving Feebas will be impossible in Pokemon HeartGold and SoulSilver.