Yes, only two such crosses have been awarded, the first one to Marshal Mannerheim himself and the second one to Finnish Chief of the General Staff, General Erik Heinrichs. Neither one is for sale, and as far as I know, they are the only 1st class crosses ever made.
There's also an extra challenge for collectors not living in Finland trying to acquire a Mannerheim Cross. The Finnish law considers them as objects of special historical value, and if you are planning to take such a cross outside the country legally, you would need a permit from the government. I imagine getting one would be a challenge.