Hey if you are in Melbourne there is a place in Baccus Marsh that makes and sells fire bricks, refractory clay etc. If they dont have blankets they will know who does!
Also there is a place just off Frankston Dandenong Rd if you are on that side of town.
When you are finished building the best oven tool shop is in Coburg on High St near the corner of St Georges Rd. He has all the toys you need.
A blanket is only one insulation method, there are many alternatives. For some interesting reading google search "rado hand" and "oven" you will get a link to a guy in Queensland who gives away plans for his oven design and 300+ step by step photo's. He does NOT use a blanket.
Personally I bought a kit from
Pizza Ovens, Wood Fired Ovens, DIY Pizza Ovens, Pizza Oven Hire, Brick Ovens, Perth, Western Australia :: because I am a big whimp! It came with a blanket (along with the rest of the materials)
good luck
cheers