One method that doesn't require a P-class or special accomodation by the DM is to get your hands on a ring of animal friendship. With a 12 HD cap you could get a dire elk dire bear, or elephant, which all have over 100 hp. For something a bit more unusual, you could have a dire bat, dire hawk (providing you are fairly light), dire snake, giant constrictor, or giant crocodile. Of course, if you wanted to go all out, you could get a second ring and have a dire elephant, dire tiger, or one of the legendary animals.