Locally bred guppies from different people would be a good place to start. Though don’t expect to make much money raising fry with one tank. With one tank your looking at the majority of the fry getting eaten by the adults.
though I have noticed in heavily planted and When more than one batch is dropped at the same time less fry get eaten. The first batch get eaten the worst. Same when you have fry a couple of weeks apart, less fry get eaten from the second batch.
Convicts would breed in a mud puddle if you let them...lol. Definitely a good starter for breeding. Just be careful. It's easy to get overwhelmed once they start popping. You can get a new batch of fry every 2-3 weeks.