www.eBay.co.uk is tops for finding that stuff cheap, I got my basic set of webbing for about $100 or so. Just stay away from Webtex and you'll get some quality kit
*edit* If you do get the webbing, you can use the 'brecon pouch' for dumping your mags, simply unzip your smock a bit or unbutton the top of your shirt and drop the mags down your chest till you have downtime to retrieve them.