Cf. [Sophocles Oedipus Tyrannus l. 110-11] ṯὸ δε ζηṯούμɛνον ἁλωṯόν, ἐκϕɛύγɛίν δεṯἀμɛλούμɛνον, what is sought is found; what is neglected evades us. A more direct source is matthew vii. 7 (AV) Ask, and it shall be given you: seek, and ye shall find.
He that wyll seke may fynde And in a brefe tyme attayne to his utterest desire.
[1530 in J. Palsgrave L'éclaircissement de la Langue Française A5]
Serche and ye shall fynd, in every congregacyn that long [belongs] to the pope.
[c 1538 J. Bale King Johan (1931) l. 192]
‘Seek and you shall find’ does not, it seems, always extend to that [health] of the body.
[1783 J. Jay Letter 14 Nov. (1891) III. 95]
There is danger in the saying, ‘Seek and ye shall find’.
[1980 R. Collins Case of Philosopher's Ring xiii.]
Related to: diligence; wanting and having
Bibliography of major proverb collections and works cited from modern editions is available here.