I've checked newegg, amazon and eBay but the reviews for most are mixed, vague or non existent. Does anyone have experience with some of these to point me in the right direction?
A small search in the forum would bring you many dicussions about changing atennas.
Some time ago I started this thread about replacement atennas for my N66U device - the discussion there is still valid. I am still happy to have replacemented the middele antenna with the 5/7 dBi directed beam vs. the original 2 dBi omnidirectional antennas. Remember: More dBi gain means more focused beam!
I did the same thing with my AC68U device with no measurable improvements...
Looks like there is still a mile to go for Asus to improve the power distribution of this device (compared with the N66U with Merlins "em-build" firmware).
you can use higher gain antennas which would help if you are going over 100mW transmission. Wireless devices have a transmission power limit so if you are transmitting signals more powerful than they can send back than you will need higher gain antennas. I have seen 1W wifi APs before and they use very big high gain omni directional antenna so that they can recieve laptop wifi which usually peaks at 100mW or up to 500mW for some cards from much further away.
Other than that you can use directional antennas to increase reception at a particular direction compared to omni or you can even attach a dish for point to point wifi.
The stock antennas are fine but if you want better you can try getting ones that look similar. The stock antennas are low gain omni directional not beam. A few dBs higher for antenna might increase range but i would not rely on that.