I would go for either of the following editions:
* _Le Morte d'Arthur_. Ed. **John Rhys (1906)**. (Everyman's Library 45 & 46.) London: Dent; London: J. M. Dent; New York: E. P. Dutton. You can find libraries that have it through WorldCat.
* _Le Morte Darthur_ : Sir Thomas Malory's Book of King Arthur and of his Noble Knights of the Round Table,. Ed. **A. W. Pollard (1903)**. 2 vol. New York: Macmillan. This seems to be the edition that some websites provide for free:
* on archive.org: volume 1, volume 2.
* on Project Gutenberg: volume 1, volume 2.
* In a version available on Full Text Archive, A. W. Pollard writes:
> the most anxious care has been taken to produce a text modernised as to its spelling, but in other respects in accurate accordance with Caxton's text, as represented by Dr Sommer's reprint.