"Uchida" brand Bistro liquid chalk markers are my absolutely faaaaav and are totally compatible with our chalkboards!
I've snagged a WHOLE BUNCH of these so that you can add them onto your purchase, easy peasy, lemon squeezy.
These liquid chalk markers are opaque, water based, pigmented and eraseable with a lightly damp cloth on our professional grade chalkboard surface. They are not recommended for painted chalkboards.
Disclaimer: Always test chalk markers on a small area in advance of your project.