There are special rectangle cupcake liners avaiable to fit some of the mini loaf pans. For the tiny loaf pan (2 1/2" x 1 1/2" base), a standard cupcake liner will work. Simply fit the paper into the pan. The batter seems to rise with the liner, so rectangle will bake higher in the middle. Giant or jumbo liners may be used with larger mini loaf pans if commercial rectangle liners are not available.
Instructions and image taken with permission from Vi Whittington and Autumn Carpenter's Book, Creative Cupcakes.