Before you start, you're going to need to get some tiles. Although you could make your own, when pre-manufactured & glazed tiles only cost ~$0.25/each, I'm going to end up going the pre-manufactured route :). The tiles I used for this project were "Rialto White" Floor Tiles
from Lowe's. Cost? $0.28/ea.
Don't want to use the tiles I did? No problem. Here's some tips for picking a nice tile that looks cool when you are done lasering it:
Really light colors or really dark colors look a lot better than neutral colors. Remember that the underlying ceramic will be a gray color which only becomes noticeable if the color of the tile ceramic stands out against the color of the tile.
Shiny tiles don't work so well. My guess is that you get some reflection of the laser, and you get more "etching" than cutting of the underlying material.
If you need these to fit into something, don't trust the sizes of tiles to be perfect. The size of the tile is pretty much always overstated...you know how those marketing people work. The 4" x 4" tiles that I purchased were about 3.6" x 3.6". So, if you need your tile to fit something, buy 1 first, before you plan a magnificent Legend of Zelda fresco around tiles that are the incorrect size :).