Common sacrifice fodder includes tokens from cards like Jadar, Ghoulcaller of Nephalia, recursive creatures like Reassembling Skeleton, or creatures that reward you when they die, like Greedy Freebooter
Then, by paying and tapping Selvala, you add X mana in any combination of colors, where X is the greatest power among Creatures you control
While I'm not suggesting to just jam this in any deck, even if you don't have any inherent typal synergy with the second part of the card 3 mana (with flash as well) to give your other noncreatures flash can be really handy for anything that wants to play on other people's turns, like some artifact or enchantress decks
Its very expensive to buy a regular booster box when a new expansion drops, and sometimes you dont want to fork out $50 for ETBs or Premium Collections either
Secret Lair Sleeves feature unique artwork, and only a few codes are being distributed