Ebay has some, try as below or just search.

http://www.ebay.co.uk/itm/2-X-HANGING-BASKET-BRACKETS-FOR-CONCRETE-FENCE-POSTS-TOP-QUALITY-UNIQUE-DESIGN-/151727515847

http://www.ebay.co.uk/itm/4-X-HANGING-BASKET-BRACKETS-FOR-CONCRETE-FENCE-POSTS-H-SECTION-/261935416751

Different type:

http://www.ebay.co.uk/itm/HANGING-BASKET-BRACKET-FOR-SLOTTED-CONCRETE-FENCE-POST-NO-TOOLS-NEEDED-/181765946690

Different sort again!

http://www.ebay.co.uk/itm/2-X-HEAVY-DUTY-HANGING-BASKET-BRACKETS-FOR-CONCRETE-FENCE-POSTS-TOP-QUALITY-/271884441019

Ideal world

http://www.idealworld.tv/pp/growing-set---pair-of-fence-hanging-basket-brackets-339819?fh_location=//IdealWorld/en_GB/$s=hanging%20basket%20brakets