For 40K gear (in the UK anyway, although I think all of these ship internationally) - http://www.skianmhor.co.uk/40k.htm have a whole load in stock and they do custom work as well.
Wandering Soldier are also doing custom jobs.
And a few others (such as Having a larp) have been spotted sporting 40K-related stuff on display at events.
There's also a few people who do personal commissions.
Not that I've become an expert in this particular subject...