assures Shylock thatAntonio will guarantee his loan. However, Shylock personally despises Antonio, on account of the fact that the latter is a Christian and lends money without interest. Shylock believes that Antonio's practice of lending out money "gratis" lowers interest rates in Venice and cuts into his profits. So, Shylock doesn't like the fact that Antonio will be guaranteeing Bassanio's loan.

Additionally, Shylock remembers Antonio's past treatment of him and is still angry about it.

You, that did void your rheum upon my beard

And foot me as you spurn a stranger cur

Over your threshold! Moneys is your suit.

What should I say to you? Should I not say,

“Hath a dog money? Is it possible

A cur can lend three thousand ducats?”

“Fair sir, you spet on me on Wednesday last; You spurned me such a day; another time You called me ’dog'—and for these courtesies I’ll lend you thus much moneys?”

(act 1, scene 3)

Bassanio learns that there is a rich and beautiful woman, Portia of Belmont, who will give her hand in marriage to the suitor who chooses correctly from three caskets of silver, gold and lead. To have access to this opportunity, Bassanio needs money, which he is short of due to his lavish lifestyle. Bassanio reasons that his marriage to Portia will allow him to pay off his debts and begin again. Antonio borrows this money from Shylock the moneylender. Antonio has investments in the form of merchant ships that are due back well before the expiration of his contract with Shylock, so he gamely agrees to Shylock's terms.

If, as has been suggested, William Shakespeare adapted his play The Merchant of Venice from that of his prematurely-deceased contemporary Christopher Marlowe’s The Jew of Malta (also known more formally as The Famous Tragedy of the Rich Jew of Malta), then the answer to the question—why did Antonio and Bassanio approach Shylock for the loan rather than some other moneylender—lies in “the Bard’s” fealty to his source material.

Both Shakespeare and Marlowe’s plays are highly prejudicial towards Jewish people, replete with often crude stereotypes common to much of history. Historically, Jews were prevented by majority non-Jewish (often Christian) populations from serving in most professions. What little was left to Jews as a means of financial support often involved finances, including moneylending. The stereotype of the Jew as pernicious moneylender, then, took root and found its literary apotheosis in Shakespeare’s The Merchant of Venice. If Shakespeare, inspired by Marlowe, sought to build the dramatic half of his play (in contrast to the more comedic half, that involving the competition to win the fair Portia’s hand in marriage) around the issue of a barbaric financial arrangement, then, it was only natural that his antagonist would be a Jewish moneylender. After all, let’s look at Marlowe’s early description of his “Jew”:

“The story of a rich and famous Jew who lived in Malta: you shall find him still, in all his projects, a sound Machiavelli; and that’s his character.”

Now, could Shakespeare have had his protagonists approach a moneylender other than Shylock, who Antonio in particular views as the scum of the earth (In Act I, Scene III, Shylock verbally attacks Antonio, saying, “You call me a misbeliever, cut-throat dog, and spit upon my Jewish gabardine”)? Hypothetically, yeah, sure, but then what becomes of the play Shakespeare envisioned and adapted from that of Marlowe? Antonio and Bassanio’s dialogue makes clear that the answer to the former’s money woes, which are incurred for the benefit of the latter, lies in an arrangement with Shylock. The assumption is that Jewish moneylenders are the only option—that pernicious stereotype, again—and that Antonio will swallow his pride and arrive at an agreement with Shylock for the 3,000 ducats in exchange for a pound of his, Antonio’s, flesh should he fail to repay the loan within three months. Antonio is sufficiently confident and arrogant in his business dealings that he cannot envision a failure to repay Shylock in a timely manner. Responding to Bassanio’s reservations regarding such a barbaric arrangement, Antonio merely replies:

Why, fear not, man; I will not forfeit it:
Within these two months, that's a month before
This bond expires, I do expect return
Of thrice three times the value of this bond.

The short answer to the question regarding the source of funding for Bassanio’s plan to woo Portia is that Shylock is assumed to the be the most readily-available source of money (although Shylock himself notes that he will be borrowing the money from another party) and that Shakespeare’s narrative demands such a singular focal point for his protagonists’ ire and efforts at securing the needed money. For the purposes of the story, there was hardly need of injecting into the narrative a protracted debate about alternative sources of money, although Antonio does note early-on that he himself never charges interest.