Where do you get the TM Flare Blitz in pokemon diamond?

I've been trying to beat the E4 but the only fire type moves my Rapidash knows are Fire Spin and Fire Blast. Fire Blast has to little pp but then fire spin is to weak and I don't have enough money to buy any either to restore my pp and every place I go there is never any there. I've been trying to find where to get Flare Blitz because it's got 15pp and it's attack strength is 120 but when I search on the internet it just comes up with stuff like rate my team and all this other wierd stuff.