We travel to Thailand as a family, usually 6 of us, and on a few occasions there are no arrival/departure cards available on the plane which means there is even more of a delay at immigration at Suvarnabhumi. I end up filling in all the cards as it is the Mrs, Me and 4 kids. All this done among the scrum with hundreds of others who also don't have a card. Does anyone know where i can get hold of some cards in advance of my travel?

Next time you are on a plane that has the cards, as the stew if you can have a bundle of them.  Also, check the magazine rack at the rear of each section, sometimes they're put there, grab a bundle.

 

And, next time you go through Immigration at Swampy or Don Muang, ask for some extras.

 

Mac

There's usually a rack of them before Immi control (as Thais have to fill them out when leaving).

 

Probably better for the OP to fly with his family on future trips to Thailand with an airline which does provide TM6 forms for completion on the plane as a matter of course, rather than with one which does not!