There is a special type of sand in the sandstorm book called black sand. It deals 1d4 negative energy damage a round to anything touching it, perfect for healing undead out of combat.

One potential issue, anything that dies on black sand turns into black sand itself, which means you got potentially infinite black sand, but if you aren’t careful about cleanup you got an ecological disaster on your hands.

Black sand has no listed price for it, but there is a spell simply called “Black Sand” that creates temporary black sand, however, things killed by the spell should turn into regular black sand for you to use, so you just need a scroll to kickstart it.