Depends on where you are based but most builder's merchants could do a better deal for you on treated decking timbers if you buy the lot in bulk, alternatively you could try a local sawmill, a lot of these produce fencing timbers anyway and some now specialise in decking material. Don't forget you will probably be building small pads in blockwork to support your deck, so you'll need sand, cement, gravel and 4" blocks/ stock brick as well. The most important part of the whole process is careful planning to minimise waste and optrimise the lengths of decking timbers you can use. Levels and setting out are also critiical but once you've established these the deck should go down in no time, weather permitting of course...