You can get one from Steven at Silph. Co. You must do A LOT of things first. You must've defeated the Pokemon Leauge and Red. Then, get the Kanto starter from Oak. In Saffron city, go to the Copycat girl (her house is the purple one). She lost her Cleffairy doll in Vermillion City at the Pokemon Fan Club. Go there and talk to the fat guy. He gives you the doll. Get out of there. Steven sees you and talks to you about Latias (only in Heart Gold) or Latios (only in Soul Silver). Go back to the Copycat girl. She gives you a Magnet Train Pass after you give her the doll. Then (I think), go to Silph. Co. (also in Saffron City). Steven asks you if you had one stone, which color would you pick (blue=Mudkip, red=Torchic, and green=Treecko)? That's the Hoenn starter. He leaves. Go to the Pewter City Museum. Talk to him. He just talks about some rocks he''ll never see. Go back to Silph. Co. And he talks about the trade! He wants a Foretress in return, the evolved form of Pineco (it's also the pre-evolved for of Claydol).
You can catch it by placing 21 blocks in the forest in safari zone. However, you have to wait 30 days after placing it.
First get the Hoenn starter, then meet him in the fossil place, then he's back at the silph co. You need forrtress to trade for beldum stats held item dawn stone same level as the forretress you trade knows only take don. Fight Red with hitmonchan, entei,suicune,raikou,lugia,feraligatr. Red's pokemon are snorlax, the 3 kanto pokemon, last evovled forms, lapres, evovled all level 80.
You can catch one in the safari zone if you throw in the right amount of blocks in the correct area, get one from one of the pokewalker courses or trade a Forretress for Steven's Beldum. For the last option, you'll need to have defeated Red, gotten the two extra starters, talked to Steven in the Pewter muesum nd then find him again in the Silph building to trade.
Get from Steven at the Fossil Building in Pewter City!
Trade from platinum