There's a nicely done map in the old GoO book. I think it used to be available online as well, but the quick search I just did proved fruitless.
However, I did find two other maps that, while not quite as nice, may be helpful as well:
On these maps you can find the castles and strongholds of quite a few of the minor houses of Westeros. Not all of them, mind, but then I'm not sure even GRRM himself knows where exactly all of them are located.
If you want to find out which castle belongs to which house (and which region is home to which minor house), the Citadel's Heraldry page
is a good place to look.
EDIT: Link to Map 1 corrected.