Mine was bought second hand and already had the 124 gig memory thingy, which I am finding is enough atm, so I can't help on that score, but for things like I normally leave my name and number with all the local second hand gadget shops and they ring me when they get what I'm after in.
Been looking on net. The official 320gb ones are between £60 and £70. The imitation ones are £30 ish. The reviews seem mixed, between rubbish and don't work, to fabulous! Ds is saving up, so obviously the cheaper the better! Thanks.