If you do not want to keep both bread flour and all-purpose flour, buy some vital wheat gluten. Large supermarkets would carry it in the baking aisle - Bob's Red Mill, Arrowhead and Hodgson are all nationally sold brands. Add one or two teaspoons to every cup of all purpose flour to bake the equivalent of bread flour. It is also great to use with your whole wheat breads or any bread that has non-bread flour in it (oatmeal, rye, etc). It strengthens the gluten in your dough, so that you get a better rise.
If you have GFS in your area, check out their flour - smaller bags than Costco. They carry high gluten flour - only flour for bagels! - and you can mix it half and half with all purpose to make a bread flour. Or use for pizza dough or other breads. I really like it.