I bought some of the Bohin white chalk pencil lead refills and use them with an old 0.9mm mechanical pencil I have here at home. (Has to be the 0.9mm, rather than 0.5 or 0.7mm mech pencils. You can get them at any office supply, Target, Walmart, etc). They work FANTASTICALLY well. I found them at 50% off with a $1 flat rate shipping at Joann's. Unfortunately that sale isn't going on anymore. The leads are kinda spendy (for what they are) at regular price.
I also have some Roxanne white pencils which work pretty well. It's much more convenient to use the mechanical pencil though.
I didn't think to look, but you might want to see if Home Depot sells white chalk leads for mechanical pencils. They could work as well. I have also heard of people using the white marking chalk that you can get at Home Depot, spread with a sponge brush in lieu of Pounce Powder. But the Pounce powder works well too.
Another thing, put some fine grade sandpaper under the quilt top before marking, if you use pencil of any kind. This will prevent the stretch that happens when marking with pencil. In lieu of sand paper, iron freezer paper to the back before markign.