If you are new to the hobby, you may find this list of suppliers and others helpful.
Lighting, DCC and Electronics
Lighting for locos to coaches and scenic items, Express Models supply the lot - can't fault them.
Decals & Details
Making up the latest liveries requires the use of decals that you either make yourself or obtain from specialist suppliers like Precision. They make an excellent range reflecting the latest liveries running around the UK rail network - however a word of warning - always read the application and varnishing instructions!
Fox carry a huge range of decals, paints and nameplates. The bulk of their range is steam era stuff, but they do a fair amount of modern stuff, most of which is essential to anyone converting and detailing RTR items.
For Network Rail/Railtrack and BR Departmental stock I can recommend Scale Model Transfers, they offer a fantastic range within this subject.
Shawplan carry a wide range of detailing parts, kits and most importantly, nameplates.
I've just started moving on to more adventurous projects and discovered Hurst Models. They sell a wide range of modern conversion kits, mainly for coaches, allowing you to create a range of "non-standard" items that feature in engineering and operator rolling stock fleets.
Since my interest is modern image, the society to join is DEMU , they run an annual exhibition which showcases the best modern layouts and attracts many traders selling the sort of stuff you need to model the dielsel/electric scene from the 60's on.
A great forum for railway modellers is RMWeb, stacked full of useful advice and v friendly people, look out for me, I'm "Stroudie".
I particularly enjoy getting out and about and photographing the real railway as an inspiration for projects. However extra reference photos, particularly of hard to find stock, are always essential. One of my favourite websites is Nathan's Rolling Stock at http://rollingstock.fotopic.net/.