How many of us when we go to make good metal templates can`t get good stiff card. Where do you get it in good sizes..remember card not paper. I would prefer a few 8x4s of it.
The plastic stuff is great - imagine it's more costly than card. Hitch has made me think..try those places that stock laminates, (you know, formica and the like) maybe they have some similar protective layers. Even B&Q type places - all those stacks of sheet material, see if you can spot anything likely in the layers, they might be happy for you to take it away (saves having to pay to skip it). Maybe even try one of those materials exchange places, our local one is Eastex.org.uk, like freecycle for businesses. you could post a wanted ad.
An old ( arty type) girlfriend of mine, used to get the stiff 'model makers' card from the Art shop. she used it for making small mock ups of the theatre sets (where she worked behind the scenes) Great little models, a la 'Jonathon Creek type' things. From memory the card seemed to be twice as thick as a cornflake box. Give it a try, I'm sure they would at least know where to get it even if they don't stock it.
Lion do art materials, they have a 2mm card ('Grey board'), 40" x 30" which looks to be the cheapest, or thinner 'smooth face pulp board' 44" x 32". (Both under glass/frame backing materials). On the whole, I'd have thought art board was rather pricier than industrial packaging type stuff but if you need it then it saves messing around going on the scrounge.
Yeah - I love it too but sometimes it's necessary (and nice!) to slap your dollars out and get next day delivery of shiny new things. If it's something for my own work I'll spend days/weeks (!) searching around (if I don't have a deadline to meet), for a client I usually have to have it yesterday.