Guppies breed ridiculously easy and grow out faster with less work, but their adult size isn't much of a snack for an aro...
Maybe mollies or platties?
I put 6 guppies in a 30 gallon tank last April. I now feed 10-12 adults every week to my oscar. There are so many babies in the tank, I almost wish I had used a 40 or 50 gallon tank. It took me about 6 months to get to where I could pull more than a few adults out a week, but now there is a very steady supply.