Came across these the other day, fairly sure they are made by one of our members, two stacks of 7 shots with a twisty top to swap between the stacks and cheaper than the normal 7 rounders. They state that they are planning 20 and 30 round mags as well. Only downside appears to be that you have to do a non permanent mod to your TPX but for the price could be worth converting over to these especially if you only have 1 mag for yours at the moment since you get twice the capacity for the same(ish) cost:
http://www.flpb.co.uk/twistmag.html
http://www.flpb.co.uk/twistmag.html