Roti's link is the right place to start and learn.
i've found over time that i can't get close to the BIR taste using "made" powders for curry powder and tandoori masala (any of the branded powders are what u need - i buy rajah, Leena, TRS for example).
i do think it essential to make your own spice mixes though (listed in Roti's link, typ for me in order that i've used them - LB, Kushi, BE and Derek Dansak's).
i think with a bit of experiment u can also successfully make your own garam masala (i've not actually tried the bought though).