You may wish to discuss your situation with a Nevada lawyer, so that you can get specific advice and information under Nevada law. In general terms:
Quote Quoting nickcindy
We are planning on taking our roommates to court but I don't know if I have to wait until the lease is up or can I sue as soon as possible.
You should be able to sue now, as you can reasonably calculate your damages based upon the remaining months of the lease.
Quote Quoting nickcindy
Am I obligated to find a roommate to replace them if I take them to court.
You may have a "duty to mitigate", which would involve demonstrating that you made a reasonable effort (successful or not) to find a suitable replacement tenant. (Note that if you can only find a replacement tenant by lowering the rent, you would be able to claim the differential as damages.)