Originally Posted by
Nihilan
Instal the sharedmedia library (no space). You can get it from Curse, Wowinterface, Curseforge, etc. It patches the standard media library file used by mods and adds a shitload of texture assets that can be used in your UIs. It may well include the one you are looking for.
But to be perfectly honest, that just looks like a simple gradient effect, which most frames can probably do without textures. Im not familiar with that particular UI though. If it is a Lua based setup, they tend to lack customization options.
I will add though that if you are determined to recreate the image on the left, you can do it exactly using Shadowed Unit Frames.