Try RSOV.com
You can get ones like JG or Dboys for around $8 each, they are sometimes on sale in a 2pack as well which can help you save $. But don't go here if all you need is a few mags, think about shipping.
Don't waste time with TM mags. The body might be done a little better, but the components inside a clone mag, are the same ones in a $40 TM mag. So yeah finding good deals on mags can help you save alot. If a mag breaks, it's usually due to the winding wheel with the large flat spring in it. Mags can easily be taken apart and fixed. You can get the replacement spring wheels on EhobbyAsia.com
EbayBanned.com and PointAct.com also offer some good deals too.