I have seen them for cheap ($8) in those cheapo shops that sell all sorts of everything - don't think they are a chain but are always a real treasure trove of things. If you want to do one yourself you can always get a cheap photo frame and put in a background and use as the whiteboard and a baking tray (can decorate it as you like) as a magnetic board. Or sometimes you can find metal photo frames which will then work as both. The post shops had some different colored magnetic/white board squares you could arrange as you like, which look quite good. Officeworks and Riot do sell but are quite expensive.

(I have looked into this a fair bit, because we live in a small flat so things which are dual purpose like white board for me/drawing space for DD are great - and for some reason is extra exciting for her to draw on that like mummy :-) )