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.