Though Apple wants you to pay for every ringtone you want to have on your costly(as in India) iPhone 3G via iTunes.But here’s a way you can directly upload ringtones to your iPhone.
First of all you must have your songs which you want to upload.You can get your favorite songs from sites like Masti4India.After this you must get iPhone Ringtone Convertor from DVDtoiphone.net .
iPhone Ringtone Convertor by DVDtoiphone.net is a free utility which allows you to convert , create and edit rigntones for your iphone.It also then allows you to easily upload the tune to your iphone easily.

iPhone Ringtone Convertor supports three format, mp3 , ogg , wav to be converted to iPhone Ringtone.The iPhone Ringtone Converter also supports adding cool sound effects to your ringtones, fade in and fade out your ringtones, and make ringtone for the iPhone with accuracy up to a hundredth of a second.
After this you can upload the converted ringtone to your iphone via the same software.Remember you must have iTunes installed on your computer to upload the ringtones.
You can read a detailed article on iPhone Ringtones Converter here.