Almost every salon provides these for free or at a small cost to their customers. It provides the most obvious way for them to see their results. They put them on the same spot every time they tan, providing a virtual "tan tattoo" of untanned skin under the sticker.
Most salons provide at least 4 different stickers, and many provide many more. Your customers will love them. With 1000 tattoos per roll, your cost is well under a penny per sticker.
They are also a great buy for home tanning bed owners, since they cost so little and a roll is enough to last for a few years for you and your friends. Compare tan lines!