Find correct forms of a word
Out of curiosity, have you made any effort to quantify the accuracy of spaCy?  The way that it performs lemmatization leverages the syntactic similarity hypothesis, and as such 100% accuracy is an unrealistic expectation.

With that said, if there is a particular subset of words that are essential for you correctly classify, and you're willing to create a labeled training set, spaCy allows you to train custom models. I believe that you are able to train these models based on the existing language models, and so the amount of training data need not be prolific.