You just have to buy it at Goldenrod City Store.
I heard from a friend you just can buy it in goldenrod city department store. I'm pretty sure that's right
TM Focus Blast is TM52.
No, only Lugia can learn Shadow Blast and that's only in Pokemon XD.
Yes you can buy the Focus Blast TM move in Pokémon Platinum.
Thunder Punch, Focus Blast, Eruption, Blast Burn
Your moves are very good. But for focus punch instead you can put focus blast. Instead of flare blitz you can put blast burn or a non fire or fighting type move.
My Typhlosion has Focus Blast, Hidden Power(water), Eruption, and Solarbeam/Blast Burn.
no
Wellspring cave (Route 3)
you get it in wellsping cave
You find Focus Blast in Wellspring Cave.
it is hidden somewhere in wellspring cave