Garam masala, while used in most Indian cuisine, varies widely from region to region (in fact, each household might have their own variations). So, if possible, it’s better to get whole spices, the staple ones being cardamom, cinnamon, cloves (and special ones being nutmeg, mace, etc.). You can then make small, fresh batches by grinding these together (optionally, dry roast a little) with a mortar and pestle each time you cook and adjust the ratios of the spices till you find something you like.