Where to buy a magazine extension tube for a Remington spr 453 AKA baikal mp 153?

"Try gun shops, gun shows, pawn shops, want ads"- rebuttle: this is an extremely hard part to find. It would not be at any of those places (with the exception of maybe a random gun show) ; could people only answer the question if they actually know where to get it? I.E. you've seen one for sale somewhere!