I have not seen the plastic 5rd magazine, but you do get the metal ones, albeit scarce. They were made for rifle grenade ballistite rounds. I have a few different variations. I've also seen guys cut down standard 35rd magazine to make them into 20rd, 10rd and the 5rd - these have sometimes been sold as original 'scarce' models.
If you need a 5rd one, I would say the easiest way of doing so would be to cut down a 35rd one, including the spring and just make sure to leave some metal for the folding tabs at the bottom.
The one in the pic below is market O5 - some of the others are marked with an X followed by a number, for example, X4. In the pic below I've included the standard 35rd and 50rd (referred to as the panic mag) for comparison.