M4 20mm RIS Rail Mounts?

[xAlanAttackx]

Where can I get one to go on my GR-16's Foregrip. I want to put a Vert Grip Underneath but G&G's Rail system will not fit a Vert Grip.

I need something to fit the holes underneath. Not sure if they are a specific size.

I will settle for a whole new foregrip (preferably a Sopmod Foregrip) but I dont want it to be too expensive. I dont have a consistant income