Assuming that fessie is in North America, I would not recommend curry powder as a good substitute for garam masala. The two blends tend to be very different as sold here in terms of both the component spices and the proportions, despite their common origin - the curry powder here has lots of turmeric which tends to make the whole dish turn bright yellow, whereas garam masala tends to be a murky brown and lends no colour to the dish.
I guess it’s a bonus of living in a city with a very large South Asian population, but I’ve always managed to find garam masala in most of our supermarkets (sometimes I even get lucky and find a whole aisle with Indian snack mixes, which I am a huge fan of). If you have no luck with the grocery stores in your area, I’d second the motion of checking specialty health food stores or a specialty retailer like Penzey’s.