A good source for brass wire is Detail Associates. They have a wide range of sizes available, from .052" down to .006". They also have a limited range of stainless steel wire available. K & S Engineering also has brass wire and structural shapes available, along with brass and aluminum tubing. Styrene tubing and rod is available from Plastruct or Evergreen. I'm not sure how much of this is useful for N gauge: I'm in HO.
Hope this is of some assistance.
Wayne