Isolated usages of unus as an indefinite article have been identified in Old and Classical Latin, but generally speaking unus and ille did not establish themselves as articles until Late and early Medieval Latin.
Regarding unus, Harm Pinkster provides several commonly cited examples of unus as article or article-like from the 4th century and earlier:
Petrus vero sedebat foris in atrio. Et accessit ad eum una ancilla dicens [Vulgate, Matthew 26:69; "... and a servant-girl came to him..."]
Ibidem una aderit mulier lepida tibi savia super savia quae det. [Plautus, Pseudolus, 948; "... there will be a charming lady with you..."]
Itaque cum maesti deliberaremus quonam genere praesentem evitaremus procellam, unus servus Agamemnonis interpellavit trepidantes [Petronius, 26.8; "... when one of Agamemnon's servants..."]
Pinkster concludes, however, that "unus never developed into a real article in the [Classical] period."
Ille and unus in Late Latin
The Peregrinatio, dated to 380 or so, is often seen as one of the earliest examples of a text extensively using ille and unus as articles.
Edward Bechtel cites several examples of both. First, ille:
ubi sex tamen montes illi inter quos ibamus aperiebant
deductores sancti illi qui nobiscum erant
ille [mons] medianus tanto altior est omnibus illis
He lists many examples of ille as the article and as the pronoun, allowing comparison.
As for unus, he gives two examples where it "evidently" does not have "the force of a numeral":
ut dicretur etiam psalmus unus pertinens ad rem
dicitur unus ymnus
However, Harm Pinkster disputes the last example, saying that interpreting unus as an article there is unnecessary.
An Early Medieval example
Greti Dinkova-Bruun (ed. Clackson) provides an example of both in a brief passage from Hygeburg of Heidenheem's Vita sancti Willibaldi, a late 8th century work, and says:
Clearly, in the expression unus homo de Ispania “unus” is superfluous and the correct translation should be “a man from Spain”, while the phrase ille nautor in cuius naue fuerant means “the sailor in whose ship they had been”.
- Bechtel, "Sanctae Silviae Peregrinatio" in Studies in Classical Philology, Volume 4, page 144. This work includes the full text of the document, and his analysis of ille and unus includes many references to the particular lines where they appear, allowing analysis of context.
- Clackson, A Companion to the Latin Language, 301.
- Pinkster, Oxford Latin Syntax, Volume 1, 1114. Emphasis in original.