Corflute is what the sign makers use in NZ it's another name for coroplast. It's easy to find. I went to a signmaker and asked if they had any big campaign or real estate signs they were throwing away as I wanted to make a guinea pig cage base.
I was given my pick of signs at no cost. The labels on them had gone on wonky, it took time to peel them off but I had perfect corflute underneath. You can cut and join with tape to get the size, shape you want.
Grids are a whole 'nuther story, I've looked for a year and not found anything suitable, in Wellington and Kapiti. Doesn't mean they don't exist just means that I haven't stumbled on them, however I did find a plastic coated wire company in Paraparaumu who will make up any size anything that you want. I googled and found them. There is a similar company in Auckland.
Please let me know if you find grids anywhere!
What some people do is buy from the net and have the grids shipped in.
I am currently working with a company to see if we can stock and offer c&c cages in NZ.
You can use anything on your cardboard to waterproof it, if you go that route, flooring options like lino, vinyl etc
Some people use perspex as a "grid" barrier, if you go this route make sure you have bottom level ventilation holes so the air can circulate properly.