Textual indicators abound for the eternal and immutable nature of the Torah. Besides examples provided in other answers, here are a couple more examples:
Even all that the LORD hath commanded you by the hand of Moses, from the day that the LORD gave commandment, and onward throughout your generations (B'midbar 15:23).
And thou shalt keep His statutes, and His commandments, which I command thee this day, that it may go well with thee, and with thy children after thee, and that thou mayest prolong thy days upon the land, which the LORD thy G-d giveth thee, for ever (D'varim 4:40).
Of old have I known from Thy testimonies that Thou hast founded them for ever (T'hillim 119:152).
All this word which I command you, that shall ye observe to do; thou shalt not add thereto, nor diminish from it (D'varim 13:1).
Another approach, mentioned by the Rambam (Moreh N'vuchim 2:39), is that the perfect nature of the Torah precludes the possibility that it could be replaced with something that would necessarily be less perfect. Regarding this nature of the Torah, the Rambam cites such verses as:
The law of the LORD is perfect (P'salms 19:8).
More broadly speaking, other verses indicate that the Almighty's will is immutable and that He does not change His word:
G-d is not a man, that He should lie; neither the son of man, that He should repent: when He hath said, will He not do it? or when He hath spoken, will He not fulfill it (B'midbar 23:19)?
And also the Glory of Israel will not lie nor repent; for He is not a man, that He should repent (Sh'mu'el I 15:29).
The Ibn Ezra (Sh'mos 32:14) demonstrates that instances of the Almighty's regret as described elsewhere (e.g. B'reishis 6:6, Sh'mos 32:14, and Sh'mu'el I 15:11) are anthropopathic, and He does not actually change His mind:
וינחם: חלילה להנחם השם, רק דברה תורה כלשון בני אדם, כמו ויעל (ברא' לה, יג), וירד (שם יא, ה), ישמח ד' במעשיו (תה' קד, לא), ויתעצב אל לבו (ברא' ו, ו). והנה שמואל אמר, וגם נצח ישראל לא ישקר ולא ינחם (ש"א טו, כט), ושם כתוב נחמתי שם, י.