HiTech Ariels in Christchurch can make antenna's to suite the job, most are grid antenna's for 24dB as the solid dishes are very heavy and bulky, plus mroe expensive.
I'd always buy one from these guys before attempting to buy any of the huge marked up retail bradn stuff like dlink etc... HiTech specialise in commercial quality antennas, I've used them in KiloWatt applications on the toughest of hill tops. Their 2.4GHz stuff is reasonbly priced compared with dlink and pc brand antenna rediculous prices.
If you want distance, have a nosey at
www.dse.co.nz as their 2.4GHz boosters, up to 500mW. Only good with some AP's like the Dlink, as it doens't accept the higher powered AP's like netgear etc...
Cheers,
Gavin.