Getting original 40s civilian clothing is possible. There are a few traders out there that sell it, but they only go to particular shows and not to militaria fairs.
The shows I've found them at so far are: Crich 1940s weekend, Tutbury Castle 1940s weekend and Ramsey 1940s weekend. There's been one at the Derby Militaria fair as well. There were a couple at the Spanhoe Operation Market Garden event, but that's not going to be help again apparently.
There's also a shop down the road from where I work that sells period clothing, however it's being refurbished at the moment and hence is closed.
Here's a link to 1940's retailers
[link]http://www.friendsofthe40s.com/listings.php?id=3[/link]
Unfortunately quite a few of the retailers mentioned no longer exist. The bottom seems to have fallen out of the vintage clothing market 
Here's a link to 1940's retailers
[link]http://www.friendsofthe40s.com/listings.php?id=3[/link]
The bottom seems to have fallen out of the vintage clothing market
Must be all those worn trousers 
Stoneleigh is a militaria fair and has vintage clothing. There are a few places that do repro mens kit, best place to look is on the pozzi forum I beleive theres threads on such matter in the civi section.
