There are many such antennas on offer on e-bay. You'll need to know which standard antenna fitment your router uses (see manual).
Note, that while some antennas may offer an improvement, the biggest difference comes from having the computer (or wireless adapter antenna) and router's antenna at the correct angle or in the right position.
Raising the router above furniture level often improves reception.
Potentially some antennas will not offer the optimum load on your router's transmitter circuitry and could (maybe) damage the router.