I wrote a complete starter guide
here that you can read and get more in depth information about buying.
As for where, you can refer to the Where to Buy Vocaloids thread
here, or if yohr choice hasn’t been linked yet, I csn help you get the link.
The current version, Vocaloid5, comes with a choice of four or eight voice libraries (I believe these are the “idols” you refer to), otherwise, all other Vocaloids are bought separately.
The guide I wrote and linked above gives my general advice on who you should purchase first, but with the release of Vocaloid5, I advise now that you work with those Vocaloids until you can decide if the software is something you really want to use. If you really can’t do that, my top recommendations are: Avanna, probably the highest quality and easiest to use English Vocaloid there is, or Miku, one of the easiest Vocaloids to use plus very high quality and versatile.
I hope this answers your question well enough to be of use to you.