The Sifri is #354, in a discussion of the Berachah of Zevulun. It discusses how Rebbi Yosi met someone who made a living from Techeles, and Rebbi Yosi expressed dismay that Techeles was still to be had. The person explained to Rebbi Yosi that the Chilazon is available, but only in a specific hidden location, and when it tries to leave that location it is stung by a spider and dies. Rebbi Yosi concludes, "It is evident that the Techeles has been hidden away [i.e. and it is to be revealed only when Hash-m redeems His people]". The Arizal is in Kavanos, Inyan ha'Tzitzis, Derush #4; he explains (according to Kabalah) why the Techeles of Tzitzis was available during the times of the Beis ha'Mikdash but not today, connecting it to the Tzitz that the Kohen Gadol wore.

The Yeshu'os Malko actually addresses the problem you raised. As I understand it, his explanation is that after the Churban the Techeles was only available with Mesiras Nefesh. (He adds that at one point the Amora'im tried to get Techeles with Mesiras Nefesh -- because of the opinion of Rebbi, 38a, who he understands to be saying that one does not fulfill the Mitzvah of Lavan without Techeles. Later, they decided that the opinion of Rebbi was not Halachic, and they gave up on trying to get Techeles.) So only when there is a Mikdash is it readily available and used by everyone. Until it will be rebuilt, it will presumably remain "Nignaz."

Of course, another interpretation of the current events is that Hash-m is preparing us for the rebuilding of the Mikdash... Nizkeh v'Nichyeh v'Nir'eh!

Best wishes,

M. Kornfeld