Their's an Australian company called SimCorner that sell USA prepaid sim cards at
SimCorner- International Prepaid Sim cards
They've actually got a store at Melbourne Airport. They sell the T-Mobile $70 unlimited 4G data pack online and at the store. I purchased one online and didn't realise there was a store at Melbourne Airport and can purchase from there as well. It's located after customs in front of Victoria's Secret. I spoke to them there and they mentioned they will have a store at Sydney in the next couple of months.
Definitely great value and the sim worked perfect. I was only in NY but coverage was fine there, but they have a link to the T-Mobile coverage checker on the USA sim card page so you can type in the places you are going and check the coverage before hand.