What is the correct spelling of the singular possessive form of 'business' as in We found the business's contracts?

The possessive case (and more generally speaking, the genitive case) of any singular noun in English is formed by adding an apostrophe followed by the letter "s" no matter how the modified Word is spelled: Thus the possessive form of business is business's.

* Exceptions to this rule are biblical polysyllabic names ending in a sibilant, with Jesus' and Moses' either listed as the common examples or as the only permitted ones. Some contemporary sources incorrectly expand this exception to any polysyllabic notable name ending in a sibilant ("Dickens' novels").

Actually, many modern style guides allow the use of only an apostrophe after Words that end in S. It isn't necessarily incorrect. It just depends on which style guide you are required to use.