Darn, I always miss the flame wars... (jk)
In addition to the correct points that breeding bettas takes time and space, I'd like to add a few more.
- No, you can't leave the fry with the parents. They need individual homes. Even if you DO have room for 50-odd males and a large (3 foot) growout tank for the females, you can't keep them all forever. The containers people use for jarring male fry are usually around 1/2 gallon, totally insufficient for the lifespan of the betta. Where are you going to find people who will take 100 (or more) baby bettas, and give them a GOOD home instead of putting them in tiny bowls? The sort of male you are likely to find for free will be one that somebody has rescued from a pet store and does not particularly want - in short, not ideal breeding stock. If you want to breed bettas that people actually want, you should choose good breeding stock to start with and you are unlikely to find it for free.
- Breeding bettas is expensive. You will go through a lot of ammo-lock, a lot of water, a lot of food, and you will tie up a lot of equipment. All in all the spawning of one pair, once, can cost several hundred dollars. If you don't think you can get this much back from selling the fry, you want to think very carefully about why you want to attempt it and again, what you will do with the offspring. Creating them is all very well - accepting responsibility for their future quality of life is far more important. If you are not even willing to pay for the male, I shudder to think how much suffering you may cause trying to cut corners further down the line.
- Not every pair will spawn. You could buy a male and have him totally ignore or refuse your female. You can't keep your pair together in their own tank. Bettas are not like cichlids. Breeding takes several weeks of careful planning and conditioning if you are to have some degree of success (read some degree as the pair not killing each other). You cannot just randomly throw a male in a breeding setup with a female and have fry in a few days. Bettas are touchy, and have been made more so by inbreeding, and it can go wrong at every step of the process as I have repeatedly found out. After months of trying, several hundred dollars worth of fish and equipment, and many failures, I still have not achieved a successful spawn. If you just want fry easily, go buy some livebearers.
Personally I think that with one female, rushing out to get a male and trying to 'make them breed' is likely to result in shredded fins and mess, and if you do get eggs or fry, I don't think that you have fully thought through what you will do with them, or the fact that you could potentially end up with three hundred baby fish - which you will need to care for, for at least three months, before they can be sold, and which you are then responsible for rehoming.
Please, please consider this more carefully before you rush into it. Bettas are not easy to breed.