re: the paraphernalia, i saw those magnets at Borders, and i'm pretty sure they only made gryffindor & slytherin ones...grr
i got one of those bust-ups & a set of 4 house bookmarks - rather flimsy but cute - from borders...ooh, & a wall scroll! those came in each house, plus marauder's map (that's the one i have), voldie picture, &...something else. borders def did way better than old navy in terms of merchandise this time around
hot topic also had tons & tons of stuff - like a dozen different shirts, plus posters & jewelery & all manner of odds & ends - including, if they're the ones i think you're talking about, keychain versions of those house magnets (but i know they had all 4, plus a general hogwarts one). if you go to hottopic.com & search 'harry potter' you get like 100 results - and some of the shirts are now on clearance!
i got into the whole trading card thing & bought whole sets on ebay. those are definitely plentiful. umm...ooh! i got a bunch of poster/sticker/coloring/etc-type books from past movies, some of which i'm selling. if you'd want something along these lines i can check what i have & give it to ya for dirt cheap ;)
ooh, & puzzles! how could i forget? b&n, borders, & target all had those lenticular (read: seasick-inducing) 3D puzzles...and i now own 3 of them. *sigh* i have no self control