Railroad chalk, what's that?

Chalk for adults.  You won't feel like you're 5 years old when you use it.  Available in well stocked hardware stores or online.  Big, cheap, round, fat, and white.  In fact, it's 7/8 inch in diameter.  That's important when using your Gandalf magic everyone-shall-pass (through my labyrinth) staff!

If you really want colors then you can order a box of 72 sticks for about 14 bucks.  http://www.forestry-suppliers.com/  (park rangers must use a LOT of chalk!)