I don't know of any commission based places/people that would be available to let you work on the costume with them as it sort of changes the format of the order from a commission to something more along the lines of a sewing workshop. However, I would recommend perhaps checking in on Australian cosplay groups on facebook as people seem to be a lot more active over there, for someone willing to accomodate to your request. There is also the marketplace here on cosplay.com however, since this site is mainly US based, I would recommend the Australian cosplay groups first.
As for a price range, looking at the reference images I found when typing "Tau Fire Warrior" into google, you're looking at a rather hefty price due to the complexity of the costume as well as the materials that will be bought to make it. Basic, simple cosplays consisting of fabric (sailor moon for example) cost around $100 for a good quality outfit at best, so if you're looking for a commissioner who is going to do a good job on the costume, you may want to prepare your budget, and maybe look into commissioning certain pieces that are a bit too complex and have a go at creating the rest by yourself.
If you do look into creating it yourself, I recommend using Worbla for the main armour pieces if you can afford it, or basic craft foam for a cheaper alternative. Worbla would be much easier to use once you've worked with it a bit as it's heat mould-able making it easier to create the small details such a grooves on the armou, however craft foam can get the job done too! There are heaps of tutorials for both worbla and foam armour on the internet to help you out, and it never hurts to message a cosplayer who has worked on cosplays consisting heavily on armour for advice. Just make sure to ask them specific questions as oppossed to vague questions (how did you make your costume) so it's easier for them to answer. Also, the construction forums on this site can do wonders if you need some advice for what type of material you should use, helmet tutorials, etc...
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