Okay, first thanks for all y'all's help! Your responses were tremendously helpful.
Second, I found the belt! There's just one problem now: I can't get the thing on! If you're looking at the mower from the front, the spindle that's located more toward you and toward the right of the mower (the stationary one, not the one that engages the blades) has a guide pole next to it to keep the belt from slipping off of the spindle. However, I don't know what to do to move that pole out of the way.
Am I just not strong enough to loosen the nuts and take the spindle off, because I tried that to no avail.