It depends on where you are showing. If its a local open fun show then any halter will do, because its more on inhand work, then a halter class so if you wanted a "show" halter you could go for the stock horse type ones with all the bling, or try an arabian halter either the fine metal ones with brass, or the eyptian/beaded ones.
If you have a very typey arab see if a Arabian Society could give you some pointers on how to show him/her properly becasue its not like showing stock horse's where they stand square and look calm.
Its more of a high head, spirited look, with a more stretched out body with feet still some what square.