There are several places where you can look for a reliable staff augmentation vendor:
your business network that can give recommendations
search engines
B2B platforms
LinkedIn
When you have a short list of potential IT partners, you need to make sure, the vendor satisfies your requirements of time zone, talent pool, workflow processes, ease of communication, cultural and language fit, and security protocols.
Base your choice on these things, and also make sure to choose the agency, which offers a comfortable, not redundant process.