What's wrong in buying timber from local DIY stores or Builders Merchant? There are the number of custom timber services if you are looking for a quick fix and you can get proficient with power tools and want to spruce up a room with a view of the short term that may sell your property then you will not want to spend too much.
However, if you are looking for profit from your work or require an expert to come in and design something that can add real value to future years only as good as the wooden deck was built.
You should speak with a company that can supply high-quality wood for you to build your project and also build by them. If you are searching online for custom timber then you can have a peek here https://www.craigtimber.com.au/timber.html.
Also speaking with timber merchants whose main business is selling wood. Some companies will sell the wood to give them the best margins and do not provide the best wood for your needs. So do some research first?
The deck should nail or screwed it together. You can use one, however, if it is the longevity and quality you are after then make sure the frame screwed together with strong outer screws and deck boards, etc. also screwed with the right deck screws.
If deck boards made of wood such as heat-stable modified softwood, cedar or hardwood you will be able to use screw finishing trailhead which has a very small head and so almost disappeared on the surface. Nailing faster and cheaper, but because of the natural movement of the wood, they will not hold as firmly as screws and the structure and the board will be loose. Nailing down deck boards will also tend to split the wood and damage the surface.