Look for a well-reviewed app developer with a wide variety of projects and who speaks Ukranian or Russian. Ignore rankings and success scores. Invite her/them to a project and request proposals, not prices or bids. You want them to describe their approach to your work, and their methodology, how they would communicate with you and your developers, what the output of their tests would be, how to check the reliability of their results.
Specify that you are seeking an independent tester/evaluator, not an employee or remote developer. You do not intend to hire the individual as an employee or developer, because you want objectivity. They'll tell you what to pay.