What is the best website to buy international plane tickets in Indonesia?
Depends on how you want to pay.
If you want to pay cash or bank transfer in local IDR (Indonesian Rupiah) then Future.Travel may be a good option for you. Same if you want to pay with Bitcoin or with a credit card.
There are other local agencies like Travelnoka that work well. If you are doing a complicated trip then you should test the customer support from the site before you purchase. Give them a call or a open a chat with them. If they can support you before the purchase they most likely will answer the phone after the purchase.