I don't post here much, but when I saw your question I had to reply because I had the same problem about 6 months ago. Giant whiteboards are expensive.
Anyways, if you're interested DIY
Or
This is the biggest one on Amazon
A little over $100 (and it's not the size you're thinking of either) and when I needed one I needed about 10 of them. I wasn't going to spend over $1000 on whiteboards.
Good luck!