Bitcoin is Divisible into 8 decimal which mean you can buy Divisible unit with any amount
Are ALT coins the only option?
If you are good on trading you can choose buying Alts and play with it to make some extra Bitcoin, Ask yourself you can play with trading or not
How do I know what I'm buying is legitimate?
First of all there is one question Where are you from? As I am answering from US yes we have bunch of exchanges you can findout you are bounded with a legitimate institution to buy or sell Bitcoin.