Most of us have built, at one time or another, plastic wagon kits. Almost all of us railway modellers will have RTR wagons. These generally have one thing in common, poor representations of the brake gear, particularly clasp brake gear. More recently, and Hornby leaps to mind, wagon underframes are beginning to improve, with the latest GWR horsebox having exquisitely detailed brake gear.
Now, unfitted wagons have quite simple brake gear and kits and RTR examples are usually good enough. Sme wagons had a rod connecting the left and right actuators but not always. Some wagons only had brakes on one side - these designs would date from the late 19th century - the Board of Trade made it mandatory to be able to apply brakes from either side of the wagon, early in the 20th century, for safety's sake.
It's when we start getting into the realm of fitted (with vacuum brakes) wagons where things get dicey. Some observations of kits:
1) Brake parts being plastic and necessarily fine are fragile.
2) Details provided in the kit are usually sketchy - you might get lump of plastic that's supposed to be a vacuum cylinder.
3) There are usually no instructions provided as to how to rig the wagon's brakes.
So, what is the modeller who is serious about constructing good looking wagons to do? Two things:
1) Learn how the pros do it and find out what the prototype looked like.
2) Obtain the necessary parts to supplement or replace those provided in kits.
So, first, I would recommend that the modeller obtain books/DVDs on the subject. Such as:
a) The 4mm Wagon - 3 Volumes by Geoff Kent
b) A Guide to 4mm Coal Wagon Building by John Hayes
c) Official Drawings of LMS Wagons - 2 Volumes by Bob Essery
All the above are published by Wild Swan, so check them all out.
d) Right Track 13 and 14, Wagons, hosted by Iain Rice
Just a word of caution, Activity Media won't ship to North America for insurance reasons so you'll have to work around that somehow.
Next, where to get parts? Easy, there are a couple of sources I can think of but I have the following in stock from Mainly Trains http://www.mainlytrains.co.uk/acatalog/by-mainlytrainsmiscsuppliers.html:
These are designed and etched by Iain Rice who is an authority on wagons.
Here's an example of a wagon I made some years ago.
This is made from a Chivers etched brass kit. The kit comes with basically no underframe detail at all so I had to build it all. This is an example of a through piped brake van - one not fitted with vacuum brake itself but where the guard can control the train brakes, and the van brakes manually. The vacuum pipe is visible as are the yokes for the clasp brakes. Safety loops are made from wire.
So, there is no reason to slavishly stick to what the kit and manufacturers provide. With a bit of knowledge and a box full of bits you can make your wagons a cut above the rest.