From what you've described, I probably would not file suit. That doesn't mean that you weren't technically "defamed," the question is: what damages have you suffered that would merit an expensive and time-consuming lawsuit? Talk to a local lawyer to see if you have any options.
Any competent adult willing to pay the necessary filing fee can file a lawsuit in the appropriate County Circuit Court in Wisconsin. Winning a lawsuit once it has been filed and properly served on the defendant, however, depends upon whether one can prove, to the satisfaction of 10 ouf of 12 jurors, a legally recognized "cause of action" against the defendant. In order to best evaluate whether the story you related may fit within one of the recipes or "causes of action" which the law recognizes for obtaining a judgment for money compensation enforceable against a wrongdoer , an attorney would need to meet with you and get considerably more information about the relevant facts.
One "cause of action" which your situation might fit, as mentioned by another attorney, is "Defamation." A necessary core set of ingredients for a Defamation cause of action is the intentional or reckless making of a false statement of fact by one person, about a second person , to a third person or persons which harms the reputation of the second person. There are other ingredients but it is not clear to me whether this core set is satisfied in your case because your description does not mention whether anyone other than you heard the manager call you a thief. Even if it were clear that your facts satisfied all of the requirements for a cause of action for Defamation against the manager, a legitimate practical question would be whether the dollar amount of any recovery on it if you win is likely to be sufficient to motivate any attorney to agree to handle the case for you on a contingency fee basis.
Another "cause of action" which your situation might fit is "Intentional Infliction of Emotional Distress." If you can prove that the manager, knowing that you had not attempted to steal anything, accused you of theft and made you leave the store intending to thereby cause you emotional distress, you might have a cause of action for Intentional Infliction of Emotional Distress (regardless whether his false accusation was heard or believed by others). Even so, once again, a legitimate question is whether the dollar amount of any recovery on such a cause of action is likely to be sufficient to motivate any attorney to agree to handle the case for you on a contingency fee basis.
"Negligent Infliction of Emotional Distress" and/or "False Imprisonment" are other potential causes of action that might be considered. Even if one or both of them fit your facts, however, there would again be a legitimate question about the extent of the potential recovery if you win.
As a practical matter, if you do have a potentially provable cause of action arising out of this incident, are willing to advance the substantial costs associated with pursuing it in Court, and are willing to pay an attorney on a straight hourly fee or "blended" fee basis (rather than on a straight contingency fee basis) to pursue it for you, it will likely be easier to find an attorney willing to represent you. I suggest you meet with a trial attorney experienced in representing personal injury claimants to further directly explore potential causes of action and potential representation agreement terms.