Where can you buy individuual cards on the web?

I am looking for a Darth Vader (k)(s) and a Mace Windu (a)

Here's an alphabetical list of on-line retailers that sell individual cards. TheForce.net and I neither endorse nor condone any of these sites or their practices.

Card Haus
(The) Card Vault
Diverzions
Hose Head Hobbies
and the ever-popular...
eBay

There are also a number of card trading websites available that you can become a member of where you can either trade or purchase the cards you need. Darth Vader (K) shouldn't be difficult to get or trade for, since he's Uncommon. Mace Windu (A), on the other hand, is Rare and a Light Side staple, making him hard to get no matter where you look.

Do you think they would have a sith rising count dooku and jango fett?

You'd just have to click through those sites to see. The rare versions will, of course, be less likely to be available. I would think that Jango may be somewhat easier to find since he doesn't see nearly as much play anymore. Dooku (Darth Tyranus, in the game) still sees a considerable amount of play. Probably #3 on the Dark Jedi list beneath Vader and Sidious.