What a fun topic!
Where - either from theremnantwarehouse.com.au online, Darn Cheap Fabrics, Rathdowne Fabric & Remnants or The Fabric Store - all are independant fabric stores, all of them carry unique fabrics, often end-bolts from designers. If I need a lining, plain cotton or something like that, I buy from Spotlight (chain store). I mostly use Spotlight for haberdashery and patterns.
How - generally if I see a fabric that speaks to me in some way, or I just immediately love it! Then I decide what to make from it. Less frequently I go out looking for fabric for a specific project, eg if I have a wedding to attend etc.
Why - I definitely have a bit of a shopping addiction, just mine is fabric and not shoes! I also have a "thing" for silk & wool fabrics... if I see one I like, I buy it, even if I don't know what I'd do with it! I get my inspiration from clothes I see on people around me, in movies and tv etc. I often try to recreate clothes I've seen on TV, or at least put my spin on it!