You can buy fondant at supermarket, in white, or at cake decorating shops in pretty much any standard colour. I wouldn't try making it yourself, I've not seen one cake recipe that didn't refer to it as "store-bought" or similar. From what I've seen, your best bet to colour white fondant is to jab a skewer with food colouring into it, knead, add more, knead, add more, etc. though if you can I'd recommend buying it pre-coloured.

Gel colour, same deal. Cake decorating shop, maybe supermarket. It depends whether you mean the writing gel or the actual gel colouring (if that makes sense). Writing gel is everywhere, as you probably know

I can't remember exactly where you live, but there's a great cake decorating shop opposite Greensborough Plaza, Sarah's Cakes or something like that. I just bought the majority of the prep stuff for Amelia's cake there. They had buckets of fondant in a heap of colours, as well as buttercream. With a shaped cake like Lightning McQueen, I'd do buttercream instead of fondant, but that's just me. I'm a bit of a beginner and I'm scared of fondant I've just read a lot of books so I can get Amelia's cake as perfect as possible myself

ETA: I just remembered, apparently you're supposed to have a thin coat of jam or something to help stick fondant to the cake, and try not to have any gaps since it can dry the cake out. HTH.