Dealer, e-bay or any number of companies that advertise on-line.
Make SURE that you get programming instructions and up-front verification that it will work, as in compatible with the FCC number on your current fob, and vehicle model. Some units require "Dealer only" programming / set-up.
What year and location / country? :huh:
There is also a chance that the remote lock "brain" is the issue, unless the system works OK with another matching remote.
If the problem goes deeper than a simple self-install fob replacement, get an aftermarket alarm system. You'll be better off.