Adjustable Height Easels

I am wanting to build an artist's easel for my girlfriend, but am having a hard time finding this hardware. If you look in the attached picture, there is a brass strip, almost like a ladder, going up the middle of the center post. Right under the paint tray/bottom canvas support, there is a spring loaded finger pull that you pull out to adjust the tray up or down. Any ideas where I can get something like this? I know I could just make something similar out of a contrasting wood, but brass just looks good.

Help please!