A lot of the time we do 5.25 components in the 6x8 spot. The nice thing is that Kicker 5.25 components come with an adapter plate to drop right in. Dont expect any bass out of em though, or a 6x8 really. But theres way too much out there for me to just throw out random brands without more of an idea of what you want, ie your budget, what will be powering them, what else do you currently have, etc.

Go to some local shops and listen to what you like.