This is not the clearest question statement so let me try to elaborate. To get single photon sources people resort to a number of relatively complex schemes. The most common seems to be using non-linear optical elements to do parametric down conversion. Basically one generates pairs of correlated photons and then uses one of the photons as a correlation gate on the second. Clearly people doing these kinds of experiments know their subject quite well and if there was a simpler means of generating photon Fock states they'd be using it. Hence the question, why does a low pressure discharge lamp not produce photons in Fock states.
After thinking about this some, I think the reason may have to do with the fact that low pressure lamps use collisions between a "thermal" stream of electrons and the gas. The generated photons aren't in Fock states because the photon inherit the statistics of the electrons. Does this sound like a reasonable guess?