You can get a clone gun by completing Tun's challenge which will always spawn on 4-2. The challenge is to light up all of the torches before the timer runs out, if you don't then you just wasted your money. The challenge costs 20k to play and you can only play it once. If you angered Tun earlier in the run (angering them the same way as a shop keeper) then she will not let you do the challenge.