Youwill have to backup and start again, or buy another drive to intall your OS onto. I dont know of any applications that will let you re-partition a Mac drive without erasing the drive.
I kept running into this problem and finally quit partitioning my drives. It seems that a lot of users (including me), use partitioning as a way to organize information that could be just as well handled by good folder use. Without partitions, Everything seems to run fine. note, that I do have a second drive for OS9 and emergency boots, so partitioning was never critical.
Note, every time I say something like that, one of the great Unix folks on the board will chime in with some strange Unix alchemy that can do anything.