Silvermans are very expensive but they have good kit, especially their footwear range.
Good suppliers (The first being the best IMO, may only be of use to UK readers):
Arktis
Kit Monster
Niton
Most of these companies, and the US equivalents, will specialise in military/police equipment, but more and more are selling PSC (I hate the term PMC) gear alongside their usual stock as there is a big demand for the police/army kit but in neutral colours (sand, navy blue etc)
Best PSC clothing is 5.11 - their trousers and shirts are the best you can buy
Cheers