That is a problem. eBay occasionally has them for sale, but as you noted, they are out-of-print now. Later this year, they are running a Kickstarter for the "Battlefields" set, which is supposed to include two maps in the larger size (the remaining two of the six total).
If you really like Ogre, then start trying to find an affordable copy of the Designer's Edition, you won't regret it.
If you really like Ogre, then start trying to find an affordable copy of the Designer's Edition, you won't regret it.