Well, I know there are several ways:
1. Search for chests in all the worlds. There should be some that carry kinda things. If you just get a bunch of synthesis items, they can be put to good use, which leads me to #2.
2. Pick a moogle, any moogle. Just go to the section where you synthesize stuff to get new stuff. You'll see that you can use your synthesis items to make power ups, defense ups, and magic ups. It's really not that difficult once you learn the process.
3. The longer, but fun way is to fight hearless and nobodies. It's a wonderful, fresh feeling to know that you leveled up in some way. If you find that it seems to be taking to long, I know a place that's great for leveling up. Go to Timeless River, the black and white world inside Disney Castle. Just go through those windows and fight hearless over and over. It's a good place to constantly fight heartless, that way they'll always come back there even when you just defeated them.
Hope that helps, Cloaked Roxas.