When you are looking for shallow planters, you can always use those inexpensive trays that are in the wallpaper department of your hardware store...the trays you dip the paper into. They are long plastic containers that come in different colors, but mainly white.
To add drain holes in the bottom, I heat a 16 penny nail over the burner of your kitchen range, holding it with a pair of plyers to avoid getting burned, then turn the tray over, and burn some holes into it.
Once you've filled them with soil and planted greens in them, be sure to pick them up toward the center of the tray to avoid buckling. Most annual flowers do well in them, as well.