It can be bought ready ground, but it does not keep well, and soon loses its aroma. It is much better to buy the whole spices, thet keep fresh much longer, and to grind them yourselves when needed. An cheap electric coffee grinder makes this easy.
There are probably as many different recipes for garam masala as there are Indian families.
(This space left blank until I can find the right notebook containing the recipe I was given.)
Please be free to add your own versions here.
Garam Masala contains black pepper, cloves, cinnamon, brown cardamom, cumin seeds, bay leaves and coriander (whole) mixed, toasted and ground.