If you can do the last boss -- Ramparts only takes about 30 mins and should be 6 shards. (Chest from the last boss drops 3 blues in Normal? Heroic is 2 blues and 1 epic)...

edit. Oops I forgot you needed large shards, normal in ramparts will only give you small. In that case, Botanica is the easiest probably. But as someone mentioned already, I'd do cache runs in Mech for your totem of the voids first. (if you don't have them already). Plus the added benefit of doing Mech or Botanica is you can get your Sha'tar rep up so you can buy the head slot enchant at revered.