When we went to the Habitat for Humanity ReStore (more info here) they had boxes and boxes of bathroom and kitchen tiles for super reasonable. Mom picked these out - they were ten cents a tile.
I lined them up on a piece of eco-felt (cork would work great too but we weren't having any luck finding any)...
Traced around them with a pen - even a regular pen in blue or black will show up on this dark brown felt.
And then I glued the felt to the tiles using some plain white glue.
Once the glue was dry I trimmed the edges...
And now she has a fabulous set of six coasters for around a dollar!
Hurray for bright ideas at the opportune moment. :)