Tree - Big W. 225cm one or something
Tree Decorations - Big W glass baubles and Woolworths plastic ones as "fillers" between the spesh ones, which are primarily from Bradford Exchange/Franklin Mint - some bought direct, but a lot via ebay US early in the year (they are significantly cheaper going the US option)
Lights - Kmart primarily. Checked Big W and Target and was really underwhelmed by their collections. For the tree (in particular) i wanted green wire so it wasn't obvious reasons, and i was happy with the quality of those lights and keep going back. i have checked ebay, and i think you have to know your prices vs size of item vs postage amounts to really get a bargain. rope lights (for instance) really weren't that cheap. Dealsdirect website does have some cheap bargains through the year if you subscribe to their website

plates etc we tend to use our normal every day stuff - and just grab red and green serviettes (or gold and silver depending on the color theme that year). you can pick up table setting pieces lots of places - or you can make them yourself (check out scrapbooking pages for ideas on handmade serviette rings/place card holders etc)

other decorations - bradford exchange, partylite, ebay ebay EBAY! you can buy some awesome things on ebay all year round, but there are some bargains to be had if you look around all the time.

i'd suggest having an idea in mind as to what you want to do decoration wise. it's awesome to shop, but always better to have an idea in mind so you don't get 30 things from 10 places and then wonder how you'll get them to work together. we tend to pick a color theme now for next christmas for wrapping paper if we can, and pick up paper that works with that at the sales. in the past it's been silver and white or red and gold. the decisions mostly comes about when we hit the first shop with decent sales and see what they have lol