For very simple candle spells use birthday candles =) they come in a variety of colors. They're awesome "on the go" candles. If I use one, I stick it in clay or dirt. These are nice for "charms".
You can also divide a normal candle into 3, 5, or 7 sections by carving or drawing lines horizontally across it in even portions. Perform the same spell everyday, allowing it to burn to the line, then snuff it out but visualize that the flame still burns (the energy of the flame remains). Continue to do this everyday, the same day each week, or once a month on a particular moon phase until all sections have been burned.