Two like these will work:
https://www.amazon.ca/TP-Link-TL-ANT2408CL-Omni-Directional-Antenna-Connector/dp/B002VYP5QW/ref=sr_1_4?ie=UTF8&qid=1526772753&sr=8-4&keywords=wifi+antenna
Here is the exact part you need. Buy 2. If that's not enough you could go crazy with high-gain yagi antennas.