save up your money.
Finding the Lego Star Wars General Grievous is not a very challenging thing to do at all. In order to find General Grievous, one can purchase his action figure at Toys R Us.
Play it
If you are talking about the episode "Lair of Grievous" from the Clone Wars TV show, Grievous's lair was on the planet Vassek which is in the Outer Rim.
There isn't one
lego star wars for 3ds is based on the clone wars.
General Grievous
the twilight from the star wars the clone wars movie
It depends on what star wars the clone wars you mean.
Maybe
Grievous isn't in LSWII, only the first one as a boss for Episode III. But, if you have a Lego Star Wars 1 save file with Grievous unlocked, you can go into Lego Star Wars 2 and buy the cheat "Use old Save", which lets you take all your old characters from Lego Star Wars 1 and play as them in Star Wars 2.
Defiantly General Grievous. He is military trained, good at fighting and very sly. Admiral Ackbar is not any of these, though he is a good thinker and can come up with useful tactics that General Grievous would not be capable of.
Lucasarts.