We don't recommend any particular exchange, DYOR*. You can use other Centralized EXchange platform to buy MATIC (ex. Coinbase, KuCoin, Kraken...).
*Do Your Own Research
MATIC is Polygon's native cryptocurrency.
You need MATIC to perform transactions on the Polygon network and therefore to perform transfers to and from the OYABUN application.
You will find all the information you need to buy MATIC by following the link below: 🔗binance.com/en/how-to-buy/polygon
You only need a few MATIC to start. 10 MATIC should be enough.