Yes you can buy aftermarket antennas for better signal.
They range from simple omni-directional antennas to overkill directional ones like a cantenna.
http://blog.codinghorror.com/extending-your-wireless-network-with-better-antennas/
Just make sure you get one for the wifi band (2.4Ghz or 5Ghz) you are using, and that it fits your router's antenna socket.