I can't find a feraligatr in gts until its in my pokedex so where can I fight one and add it to my pokedex?
Depending on what game you're playing, in HeartGold/SoulSilver, you can't fight one unless your rival chose Totadile. In Pokemon Diamond/Pearl/Platinum, there is unfortunately no way to battle a Feraligatr. Also in Pokemon Black/White/Black 2/White 2, there is also unfortunately no way to battle a Feraligatr. Try going to GTS Negotiations and trading with other people. Some people usually offer starters.
Hope this helped.