I can't find where this text comes from:
Ambulatoria enim est voluntas hominum usque ad vitae supremum exitum.
I can't find the author on the internet. What is the source of this phrase?
The phrase is actually slightly different:
ambulatoria enim est voluntas defuncti usque ad vitae supremum exitum.
For the will of a dead man is changeable until his final departure from life.
This comes from the Digest (or Pandects):
The Digest, also known as the Pandects (Latin: Digesta seu Pandectae, adapted from Ancient Greek πανδέκτης pandektes, "all-containing"), is a name given to a compendium or digest of Roman law compiled by order of the Eastern Roman (Byzantine) emperor Justinian I in the 6th century (AD 530-533). It spans 50 volumes, and represented a reduction and codification of all Roman laws up to that time.
The full passage is Dig. 34.4.4.:
Quod si iterum in amicitiam redierunt et paenituit testatorem prioris offensae, legatum vel fideicommissum relictum redintegratur: ambulatoria enim est voluntas defuncti usque ad vitae supremum exitum.
This particular quote is compiled from the writing of Ulpian (c. 170-223). I am not sure if this particular quote is extant beyond the transmission from the Digest.
This appears to have gained some popularity as a legal maxim regarding will-writing: a Google search reveals several hundred results from old legal books. The basic idea is that a will can be changed as long as the person in question is alive.