I did some looking around and AirsoftGI gave me the best deal, some places in Canada were charging 60 bucks for the 40rnd mags, wtf. Despite this I am a bit mad at AirsfotGI because they pack like retarded monkeys.
http://www.airsoftgi.com/product_inf...oducts_id=5881
You probably won't find them cheaper but I encourage you to look.