Yes, you will need a Boost SIM card, you can buy one online via the boost website or at any major retailer. For the iPhone 6 you are looking for a NanoSIM or an TrioSIM pack and to get a SIM only pack is just $2.
During the activation process for your new SIM card it will grab your number and details and will be able to bring your service from Optus over to Boost, allowing you to keep your existing number and assigning it to your new Boost SIM card.
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