Siona is an Aura commander: when Siona enters the battlefield, you get to dig through the top seven cards of your library for an Aura card and put it into your hand
However, MTG x FF products seem to be priced much like LotR , if not with higher pricetags overall, for a set that is not premium and is supposed to be Standard-legal
If you have some other crazy idea up your sleeve, you can use a card like Baral, Chief of Compliance, which works as a creature and helps lower the mana cost of Time Stretch
Filling out the remainder of our support suite, we want cards that might not be as powerful, but will at least supply incidental milling
For more info on all this, make sure to check out The Brothers War Site @ Wizards of the Coast
Its not super-efficient at finding Saheeli, but it does curve right for setting up the turn 6 one-shot kill