almost 5 years
ago -
WotC_Lexie
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Correct, the Secret Lair sleeves are not exclusive to the Secret Lair drop itself. It will be a while before you see them in the MTGA store though.
The Secret Lair cards are specific to Secret Lair, though. There are no plans to make them available outside of the 24 hour window that they are sold in. Here's the article that gives you details.