Where can I find Nagita Thera Apadana I searched in the Apadāna part of Tripitaka..But couldn't find it...This includes a story of Nagitathera's previous life, and merits Nagitathera got for the god-deeds done in that life...
Following are some pointers:


Nagita Thera

An arahant. He belonged to a Sakiyan family in Kapilavatthu and entered the Order after hearing the preaching of the Madhupindika Sutta.
In the time of Padumuttara Buddha he was a Brahmin, named Narada, and uttered three stanzas in praise of the Buddha. He was once a king named Sumitta (Thag.vs.86; ThagA.i.183f). He is probably identical with Atthasandassaka of the Apadana (Ap.i.169).

Nagita Thera

For some time the personal attendant of the Buddha (D.i.151; DA.i.310; A.iii.31, 341; iv.341; J.iv.95, etc.). He was the maternal uncle of the novice Siha, who is said to have addressed him by the name of Kassapa, his gotta name. He was fat and, therefore, lazy; he got most of his work done by Siha.

Nagita Thera

A thera of Ceylon, author of the Saddasaratthajalini. Gv.p.74; Svd.vs.1249.

Nagita Sutta

Once, when the Buddha went to Icchanangala, the brahmin householders there came, in large numbers, to pay him their respects and made great uproar outside. When Nagita, the Buddhas personal attendant at the time, told him the cause of the clamour, the Buddha replied that he had nothing to do with homage; his concern was with renunciation. He went on to state five inevitable things: whosoever eats and drinks must answer the calls of nature; whosoever loves is destined to sorrow and despair; whosoever dwells on the asubha must feel disgust for the subha; whosoever sees impermanence in the six spheres of contact feels disgust for contact; whosoever sees the rise and fall in the five kinds of attachment, must feel disgust for attachment. A.iii.31ff.

Nagita Sutta

The circumstances are the same as those of No. 1. The Buddha tells Nagita that he is pleased with monks who do not live in the village, but who seek the forest and stave off gains and flattery, but to him the best is to walk on the highway unattached. A.iii.341ff.; cp. ibid., iv.341ff.

You can try to follow the references given above.
As for the story of Nagita Thera donating a Katina robe and the merits gained, I have heard this only appears in a later composition bellowing to the Kandyan period. Therefore this may not appear in the Tripitaka, Commentaries and Sub Commentaries. Therefore, this perticular story will not be there in the Apadāna.