sphidols:

The idol support app CHEERZ sometimes has cheki events where users can win signed chekis from their favorite idols. In each event, there are 50 idols, and the top 3 fans (most cheers) of the top 10 idols after the 5 day event period win. 

It ends up being kind of pricey for just one cheki, but for foreigners who can’t attend lives, it’s an option (still need a forwarding service to mail the letters to you). These are ones I got from June’s event: PREDIANNA’s Yua and Houpri Youth’s Nagasawa Marina.
So cute! (｡･ω･｡)