When you buy your computer with the Windows operating system you automatically get some Tamil fonts that are called as Unicode in different styles.
There are thousands of Tamil fonts available in the internet for free. Most of the companies and individuals offer these for general public as a gesture towards the community. But they expect financial support form those web sites that are using their products for commercial purpose.
If you are one of the internet user and wants to read content in Tamil from those web sites that are using Tamil fonts, check good in that web site they may offer the font for you free of charge from their relevant link. Follow the instructions as how to install them in to your computer's hard disk.
Some web sites do have their fonts in their server allowing to be used when people visiting their web sites. If you are a printer or artist doing font related works or doing Tamil translation work, then it is better to purchase those related Tamil fonts from the font owners to avoid legal problems. Also do take care to buy them from the original producer as there are many people selling them with out any permission from the font creator or owner.
Also like computers many Smartphone models allow you to use Tamil font for personal typing and reading purpose. There is no standard font in Tamil for Indian people as thousands of them are available and each institute uses their suitable ones while most of the Sri Lanka Tamil people prefer to use Bamini font to read and write.