Fans you can get almost anywhere. Markets, car-boot sales and cut-price shops are good sources and, at the right time of year, you will probably see them in High Street stores. You can also find them on eBay (search for "mini fans").
We buy a whole range of wheels, gears, axles and other stuff from Tech-Supplies. They also do robot-oriented stuff used for projects described elsewhere in this website. We used to buy Corriflute from Tech-Supplies, too, but they don't seem to sell it any more (although there's still a picture of it on the front page of their website!). There are other sources on the web - it's also known as Correx - but we haven't actually used them.
TechCard we get from Hands On. Basically an educational supplier, they sell all sorts of interesting things on line and their website is well worth a browse.
Craftpacks, another educational supplier, is good for cotton reels, lollipop sticks and all sorts of craft-related stuff. They also sell simple tools, including child-safe shears which can cut card, plastic sheet, etc.