Sorry I found it

Opera developer for Windows x64 – experimental:
https://www.opera.com/download/get/?partner=www&opsys=Windows&product=Opera developer&arch=x64

Could you provide a link where to download 64-bit developer version?

Sorry my bad. Opera still have download-links available to old versions.
When I check for browser update I get feedback that it's the latest version(!)

Is Opera 38-40 available in 64-bit versions?

I've installed latest Opera_1218_en_Setup_x64 and when I go to addons.opera.com I get message:

To use extensions and themes, please upgrade to the latest version of the Opera browser!