Do you mean a customs post to stamp or a counter to claim monies for an already-stamped form?
If the latter, there is a bank counter by the base of the F pier (IIRC, right side when looking down the pier), and there appears to be a customs counter there too if you need the form stamped (but you may/will need the items to show to the customs officers). Be aware that you could run into people with lots of forms to claim back (I did a few years ago).