You stated in the original post that you started the tank back in October, so it is safe to assume that it is less than a month old. You also mentioned that you just added more livestock within the past few days. More than likely your tank does not have a sufficient amount of bacteria established yet for the additional bioload, due to the fact that it is still cycling. A large water change now will likely disrupt the cycle, as will adding chemicals to remove the ammonia. However to ensure the health of the current inhabitants you should deffinitly try to get the levels down with a small water change, maybe 25% so that it does not reduce the ammonia levels completely so the tank can finish its cycle.
I highly suggest waiting at least a month before adding anything new to the tank, and take it slow.
I also recommend running a Seachem Ammonia Alert badge on the tank so you can quickly gauge ammonia levels at a glance during cycling.
As for water, I agree that a RO/DI unit is the way to go, however in an emergency you should be able to use high quality distilled or bottled "spring water". There are two big spring water companies that run all of their production water through RO/DI (not going to plug the names), just make sure you check the labels. RO/DI is the standard though and other options should be reserved for emergency use only.
If I was buying water from a LFS I would make sure I at least had my own TDS meter to verify that they are keeping up with their filter changes to ensure your getting 0 TDS water.