Is the Summa Theologia infallible to Catholics or not? Me: Ah, so the Summa Theologia IS considered infallible! (Assuming that if the Magisterium and Pope are, then their trusted sources must be through them!) 

Catholic: No, it isn't infallible by itself, just as an individual bishop isn't infallible by himself.

I'm confused. If the Pope and the Magisterium are infallible, why aren't their trusted resources seen as such? Surely, at some point, some Pope or Magisterium must have indicated whether such a famous theological text is or is not of relevance?
No, the Summa Theologica/Theologiae is not infallible. It was written by St. Thomas Aquinas, who though he was very good, holy, and learned, was not infallible.

On those occasions on which the Pope is considered infallible (that is, when he addresses the whole Church as its teacher and pastor, and defines a doctrine which must be held by the whole Church), or for that matter on occasions when any group (e.g. an ecumenical council) is held infallible, it is not the Summa or any similar work that is ultimately the source of this infallible understanding; it is the Word of God Himself. That is the only possible source of infallible knowledge:


  At the divine command and with the help of the Holy Spirit, [the Magisterium] listens to [the Word of God] devotedly, guards it with dedication, and expounds it faithfully. All that it proposes for belief as being divinely revealed is drawn from this single deposit of faith.


(Dei Verbum Section 10 Part 2, quoted in Catechism of the Catholic Church paragraph 86)

One example in which we see this quite clearly is in the doctrine of the Immaculate Conception. This is a belief of the Church from ancient times, proposed as a dogma in 1854 by Pope Pius IX. Yet Aquinas disagreed with the idea! In his discussion of the question "Whether the Blessed Virgin was sanctified before animation?", that is, before the infusion of the rational soul, which was understood at the time to take place after conception, Aquinas says:


  In whatever manner the Blessed Virgin would have been sanctified before animation, she could never have incurred the stain of original sin: and thus she would not have needed redemption and salvation which is by Christ, of whom it is written (Mat. 1:21): "He shall save His people from their sins." But this is unfitting, through implying that Christ is not the "Saviour of all men," as He is called (1 Tim. 4:10). It remains, therefore, that the Blessed Virgin was sanctified after animation.


(Summa Theologica, Third Part, Question 27, Article 2)

Obviously St Thomas, being wrong in this respect, could not have been infallible. It was quite some time before the theology of the Church's understanding caught up to the belief itself.