Yes, quite simply: Shakespeare is not the first person to suggest that Richard III was hunchbacked. It's likely that he consulted both Edward Hall's The Union of the Two Noble and Illustre Families of Lancaster and York (1548) and Raphael Holinshed's Chronicles (based closely on Hall's work). Union melds Sir Thomas More's History and Polydore Vergil's Anglica Historia (commissioned by Henry VII) into the traditional Tudor representation of Richard III adopted by Shakespeare. Vergil's history is second only to More's in shaping the Elizabethan view of Richard III, though Vergil does make a greater attempt than More at impartiality.

Edward Hall was a judge and Member of Parliament during Henry VIII's reign. In his work, he actually darkens More's description of Richard and embellishes Vergil's reverence of the newly established Tudor dynasty. Holinshed's Chronicles closely follows Hall's Union, with some blurring and adding of details.

No contemporary writers who might have seen (or met people who had seen) Richard mentioned his uneven shoulders, but early Tudors did. More, for example, describes Richard, as compared to his brothers, as 'little of stature, ill featured of limbs, crook-backed, his left shoulder much higher then his right, hard favoured of visage'. The Richard III Society has a good overview of Richard's appearance, with quotations--you can see that the descriptions tended to become elaborate and deformed as time went on.

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Other sources:

• Dockray, Keith. Richard III: A Source Book.

• Kendall, Paul Murray. Richard III.

• More, Thomas. The History of King Richard III and Selections from the English and Latin Poems. Ed. Richard S. Sylvester.