Virgin instore were the cheapest anywhere when I bought one as a pressy a lil' while back, tbh I'd be amazed if you can find a new one now at anything other than at a stupid price or in a bundle. Best bet is would be to do a web search every day from now till Xmas on all the reserve for instore pickup sites eg Argos, Currys etc. Good luck, you'll need it.
Not too sure what one of these looks like (my kids all long flown the nest and no grand-children) but seems to be an awful lot of stuff comes up on e-bay when you put in Nintendo DS Lite for search. Hope this isn't a red herring!