They have the Zig Pens. I like Kuretake Zig Letter Pen CocoIro and use the Kuretake Zig Letter Pen CocoIro Pen Refill - Super Fine - cbeBlack: These pens are not waterproof, which makes them almost like using watercolors.
"It was a recommendation on the web from someone making their own gesso. At $19, this one costs less than half of the usual muller prices. There were also suggestions to find a glass object that had a perfectly flat bottom, like a glass or other knick-knack, maybe at thrift stores. It needs to be sturdy enough to put pressure on the glass plate."