For General Good US kit, At The Front is the benchmark.
http://www.atthefront.com/
For M-1 Helmets, restorations, and Airborne Uniforms & Equipment Top Pots is the way to go. Expensive but first rate helmets.
http://www.toppots.net/main.php?do=home&action=home
For some specific items try Soldier of Fortune, however buying things from the UK can get abit expensive.
http://www.sofmilitary.co.uk/
What Price Glory has some useful items.
http://www.whatpriceglory.com/
On Ebay Hong Kong Keith has supplied large amounts of reasonable quality gear, however his service has deteriorated in recent months, with problems with orders becoming more common.
http://stores.ebay.com.au/ww2militaria- ... esstQQtZkm
On Ebay Barry Lyndon is also a good supplier.
http://stores.ebay.com.au/Spearhead-Mil ... esstQQtZkm