most of my jeans actually have a tiny pocket inside the front right pocket, and luckily, 1 extra battery pack fits perfectly in them! lol.. But i guess buying an entire new wardrobe is probably not the most cost effective way of achieving your goal lmao...
My advice would be to get some sort of simple, felt bag of sorts... or maybe some sort of belt holster like this:
http://www.midwayusa.com/viewproduct/?productnumber=112061&utm_source=froogle&utm_medium=free&utm_campaign=655I know that its for bullets for a gun, but it would work perfectly, attach it to your belt, you wont even know it's there.. and it will keep your battery safe. Just a little pouch attached to your belt

to keep it away from coins and keys and stuff.
I wouldn't actually order that pouch from that link, I am only using it as a reference to kind of show you what I was thinking lol...