It depends what you are after.
www.mgsharp.com does a lot of low cost N gauge but then it is all imported stuff from Japan where N gauge rules dues to size constraints. You will be amazed how much more value you seem to when you look at Japanese N gauge.
If you are after British outline and already operate with Peco track then you are very limited in that it is Peco or Bachmann. Rails of Sheffield mentioned above as as good as any large stockist.
Cheers
Moped