Try a local restaurant equipment repair company or the internet maybe? I order the majority of my dishmachine parts from my local restaurant repair company.
Maybe the company that supplies the detergent for the dish machine may have it. Just some ideas.
More information would be helpful, for example; manufacturer and model, who make the pumps? If it is a perlick or CMA, they usually install their own, call them. Hobart, Champion/Moyer Diebel or Jackson has the chemical furnish them. Call the detergent company, who should replace them for free, if under contract.
"Fighting Ignorance since 1973 (Its taking longer than we thought)." The Straight Dope.