Many Ukrainian developers are working as private entrepreneurs, so you can set a contract with them directly. The taxes for them are pretty low (5% of income + social security of approximately $100 per quarter).
I'd recommend to start with the website djinni.co to search staff. That is an anonymous (for candidates) job search portal. they ask employer to pay some percent of an employee monthly salary in a case of successful hire). That is the fastest way to get in touch with candidates without paying a hiring agencies.