Home centers sell 15-amp light switches for 39¢ in bins, and 15-amp switches in boxes for $2. They'll both do the job, but the $2 switch has a beefier spring arm and larger contact surfaces. If you're just switching a few room lights, the 39¢ switches work fine and will last a long time. But if you're switching 10 or 15 recessed lights or a large motor (like a ventilation fan), or the switch gets used dozens of times a day, pick the $2 switches. They'll last longer.
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.