You have to buy it to have an activation number.
mediafire
No there isn't any vocaloid online avatar games. But there is dress up games. I also wish for vocaloid online avatar games.
You can either Google "Vocaloid Download" or "Hatsune Miku program download" That's how I found it. But yeah, It's called Vocaloid: Project Diva.
Yes, it is possible.
You can download IA Vocaloid songs from Nicosound. This site is basically the audio version of NicoNicoDouga. On this site you can download all of IA's songs that have been posted on NicoNicoDouga.
Unfortunately vocaloid isn't available as an online game, but you can play Project Diva for the PSP.
From the internet (I don't suggest this) or buy it in Japan
on thiefy.com buy vocaloid and don't steal
You install a Vocaloid like any other piece of software. Price varies depending on the Vocaloid but can be as much as $200. You have to pay for Vocaloid, unless you illegally download it. However there is a program called UTAU that is similar to vocaloid and lets you create your own UTAUloid rather then just the one voice
You can't get Vocaloid for free, but there is a programme called UTAU which is nearly the same thing; you can also make your own Vocaloids on that.
Vocaloid isn't an anime, silly. They're voice synthesizers. If you buy one you can enter a tune and lyrics to it and it sings it. They made a manga of Vocaloid though, it's called Hatsune Mix!
On the internet. Use 'Youtube to Mp3' converter and convert any song on youtube you like (i.e. any vocaloid song on youtube) so there you go :)