I would first disassemble and clean the magazine before concluding that the spring needs to be replaced. I have kept a .22 Woodsman magazine loaded since the 1950s and it still works fine. I have an early Government Model that had been loaded for 70 years when I got it and it worked fine.
If the magazine spring really is the cause (I would look at other causes first, since the first two rounds should have the most spring pressure, whereas the last two would be most effected by a weak spring), buy a new spring from Colt or Wolf or Brownells, etc..