Gatherer lists Crystalline Sliver under the Tempest Remastered set with a Block of General. I would like to purchase that specific printing but have been unable to locate any physical copies of the card. Does any one know why these cards are so difficult to find?
Tempest Remastered, Vintage Masters, Masters Edition, Masters Edition II, Masters Edition III and Masters Edition IV are all sets that were released on MTGO only. These do not exist for paper magic.
Wikipedia has a list of these sets, as a section on their full list of magic sets here.
Unfortunately, unless WotC removes their reserved list, most of the cards in these sets can never be reprinted in paper magic. Masters Editions were specifically bring these cards to MTGO, from the sets before Mercadian Masques, the other two were like the masters sets we get in paper magic, though MTGO can ignore the reserved list, which applies to only physical cards.
You can find the full reserved list here.
There was one loophole WotC had in their reprint policy for a while. Reserved cards couldn't be printed in sets, but they COULD be reprinted in other product, like promos or preconstructed decks. This gave us some of these reserved slivers in the Premium Deck Series: Slivers deck. If you wanted the Crystalline Sliver from that set due to not wanting the old card frame, you could use the PDS: Slivers copy, that would be foiled and would use the style from 8th edition to M15.