For manual white balance you need to show anything white to the cam so it gets the reference. Today I use a single sheet of paper. It works but it's not practical on the field.

I'm looking at something more rugged so I can use it everywhere. Also something with a true white (if this exists) because all the papers have different whites too.

I've heard about the warmcard system but don't need the blue card just a basic white card.

Any recomendations on places I can get that from ?

What about using a slate with color bars on the clapper?

Looking for something very portable I can use on the field when shootin documentary.

A slate would be ok but can I find one that will not produce any glare ?
Is the white used on a slate a good white reference ?

Dany,
The back of a white acrylic slate will work ok as long as (a) you don't get the clapper bars in the frame, and (b) avoid glare (as you remarked).

PortaBrace, the professional bag manufacturer, includes a 6" x 9" stiff white plastic white-balance card with many of their bags as an added "thank you" bonus.

Any stiff white piece of cardboard or plastic will do, preferably with a matte finish.