Can recommend this seller who sells on e bay as well as on his own website.
eBay My World - bmwmotormec
BMW Car Parts, Spares, Accessories, Motormec, UK
Was the cheapest place I found for a decent quality radiator expansion tank and cap.
Like all places some things will more expensive but may be worth bookmarking his e bay shop so you can see how his prices compare when you are shopping around for parts.