Three of the spices in this mixture (cumin, fennel, and mustard seeds) are easy to find almost anywhere in the US, and will create a delicious aroma and flavor all by themselves.  If you can't get fenugreek, substitute a little imitation maple flavor (which is made with genuine fenugreek).  You can get the last two spices (fenugreek and nigella) at Indian groceries.  Nigella is also called kalonji, black cumin (though other spices are also called black cumin), and black onion.  Fenugreek is also called methi.