You are in luck,
Lord of the Rings is available in Mobipocket from
Fictionwise.
Amazon may at times have an advance of a week or so over other publishers in offering new titles; but, LOR is now available in all formats. (It is even available for Sony.)
I have not seen any great difference between the DRM formatting for Mobipocket and the DRM formatting for Sony. Both are prepared by the publishers. There are good and bad in both groups. Kindle, as you point out, is just another flavor of Mobipocket -- but one specific to the Kindle. While Amazon owns Mobipocket, they do not sell books ready to read in that format, only the Mobipocket dealers sell these. Likewise, Amazon sells Kindle formatted books that cannot be directly read by units that read Mobipocket DRM files.
While many readers can render Mobipocket DRM books, only those readers offered by Sony can render BBeB books -- LRF and LRX.
If you already have a Sony 505 and LOR is available for the 505, why are you looking to get another reader that supports Mobipocket DRM?