How to approach other agencies for partnerships when you're new?

Hey everyone,

Hope this is the right place to ask. I've recently launched a tech recruiting agency focusing mostly on talent in Eastern Europe. I come from R&D myself, and we're good at finding and vetting candidates, but we're still working on building a steady client list.
I believe we could be a sourcing partner for a larger agency, but not sure how to initiate that conversation without it being another "need candidates?" email.
How did you first go about finding and approaching other firms for partnerships? Any advice would be incredibly helpful.
Thanks!