All buffets and family style restaurants at WDW are considered Table Service (TS). This means they all take reservations, and if they take reservations, then you almost always need a reservation, with some exceptions.
Catne's suggestions are excellent. I will add this to them - if you go to the podium at a restaurant in the morning, and ask for a lunch or dinner reservation later that day, they will sometimes be able to accomodate you. I've gotten into Hollywood and Vine several times with this technique during busy days when they weren't accepting walk-ups after 4pm.
The one buffet that I have never had trouble getting into as a walk-up is Trails End Buffeteria in the Fort Wilderness campground. Trails End is a little-known gem with simple, American-style food, a very casual family atmosphere, and very nice staff, even by Disney standards.
I have also been able to get into Liberty Tree Tavern for dinner a couple of times with no ADR and a brief wait. And since LTT now has no characters, it may be less popular and thus easier to get into as a walk-up.