This will give you  unlimited missile weapons for the price of 1 each:

     First  buy  a ship with 2 missile bays. Then choose a missile you
want,  and  buy  it (note you must only have 1 of the missiles in your
holding  for  this  to work) Select the missile by pressing BOTH mouse
buttons  at  the  same  time, as if you were loading 2 of your missile
bays  at  the  same  exact time. If you do this right, you will have 2
missles  of  the  same  on  your  ship, and still one in your holding,
reading x0.
     After  you  go  in  the first Karral with this, you don't have to
have  them  both loaded for it to work. So, on the x0 missles you dont
have to press it at the same time any longer.
     Repeat with other missiles you want.