I found tiles the right size for about $0.37 each at Lowe's. I grabbed a bunch, plus some other sizes for future crafting fun. I printed out a dozen or so pictures, not sure which would end up being the right size/look for the coasters so I wanted extra. I think that cost $3-4. The other materials I needed were sponge brush, mod podge, a clear protective coating - I used polyurethane, and some felt-y super slider things that I gorilla glued to the bottom of the coaster so it wouldn't hurt the furniture. I had everything except the polyurethane and the sliders - I borrowed the first and bought the second for about $5 at Lowe's.
With all the materials gathered, I cut the pictures to size, mod podged them to the tiles, covered the images with mod podge (two layers, allowed to dry) and then the polyurethane. When it was dry I attached the sliders and voila!