at Ready Nutrition I saw a bathtub bladder for a standard tub. we have a garden tub and i'm wondering if I could place 2 in there and fill and drain alternately to avoid bursting...any thoughts on this 

I don't see why you couldn't. My only concern would be weight for someone living in a manufactured home or who has their garden tub on a 2nd floor. Water is extremely heavy! 

We do have a mobile home but because the tub is so large (think kiddie pool) and im a curvy woman DH has extra support already in place. Good point thanks! We need to get info on rain water collection if u have a link. 

Check this out:
DIY Rain Barrels

Didn't realize we had not thread on this!