You can get a tourist visa from your nearest Indian consulate or embassy. It is possible to download the application form from Internet. If the consulate issues your visa, please note that it will start from the date issued, not from the date of your travel. It is recommended that you apply in advance.
E-visas can be applied for online and are valid for one month. These are validated at the airport on the day of departure.
No visa is extendable once you have entered India.