Is there any regenerating sticker source? Is it possible to leave your game in a "stuck until paid real money" state if you put pink stickers towards iron-age chests before getting ocean sculpting?
(Someone may have to confirm this for the mobile version)
In the PC version, new chests occasionally spawn after/during a thunderstorm.
Alternately, once you've colonized most of the first world, send the occasional couple dozen believers to their doom in the fiery doom pit for gems, then buy sticker packs with the gems.
UPDATE: According to comments, the mobile version does not in fact have the Fiery Doom Pit to throw followers into.