Rich,
It depends what your after. There are at least 3 manufacturers of modern colour light signals available: Roger Murry - excellant but pricy; Berko - decent, bit bulky, but cheap; or Eckon - kits, cheap, but don't look modern enough. You can find them all online, or the latter two at some model shops/shows.
As for signalling systems, you can buy them, but as said depends on your requirements. If you have a large loft layout where you 'run' the trains more than 'operate' them, then you can buy something similar to the prototype which works with block sections - i.e. when the train passes a signal is changes to red, and then clears progressively Y, DY, G as the train moves on through the next sections. This can either be done using magnets in the train and reed switches on the track, so it is very close to the real thing, or can just be done on a simple timer delay.
If you're into operating more than running, for example you have a terminus layout as many of us do, then you can't really use the above system. I find the simplist method is just to use switches in a terminus layout - perhaps with a bit of interlocking for things like shunt signals and route indicators etc. Its cheap and it does exactly what you want it to do. There are systems out there that will completely interlock and computerise a terminus layout, but I think it's just too complecated and everything has to be correct for it to work.