Hi there mate,
Aside from eBay, Soldier of Fortune (www.soldieroffortune.com) is one of main places in the UK for most things Nam. They do real and reproduction uniforms as well as webbing, protective gear and rucksacks etc. There prices can be pretty high in my opinion but they are virtually a 'one stop shop'.
In the US, Moore Militaria (www.mooremilitaria.com) are a Vietnam specific online store run by the excellent Trey Moore. Like Soldier of Fortune, he does a mixture of real and reproduction and covers just about everything. His prices are good but his shipping costs can be high, plus you run the risk of getting hit for customs charges.
Epic Militaria do a few Vietnam bits, not much, but some. They do a nice and (pretty cheap by comparison) First Pattern Jungle Uniform which I can recommend.
To round things out there are a number of Vietnam marketplace groups of Facebook (including a European one) where other collectors, reenactors and airsofters buy and sell all manner of things Nam related. I've found some great bargains there too.
Hope this helps a bit mate. Cheers, Tom
VIETNAM: Ia Drang Part 2: Broken Arrow - The Long Walk - Choker - Ho Chi Minh - Overdrive - Battle of the Chuối Gà River - Operation Diamond Head
COLD WAR: Cutthroat - Black Gold Part 2: Two Faces - Hockwerk - Double Barrel Part 1 - Operation Light House - Battle of the Weser - Gulag - DMZ 84 - The Dark
Non Gunman Events:
VIETNAM: DorkingNAM Nov. 2009 - DorkingNAM April 2010 - CS 'In Country 8' Aug. 2010 - CS 'In Country 9' Aug. 2011 - DorkingNAM Oct. 2011 - DorkingNAM Nov. 2012 - Operation: Dust Off April 2014 - Operation: Dust Off II May 2015
OTHER: CS Operation "Full Metal Wedding Jacket" June 2012 - WWII The Battle of Tulagi April 2016 - Grenada Day 2 August 2016