Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
dispŭtātor, ōris, m. [disputo], a disputer, disputant (rare): subtilis, * Cic. Off. 1, 1, 3: suae artis unumquemque et auctorem et disputatorem optimum esse, Val. Max. 8, 12 init.