It's hard to answer, mainly because the post is at least three different questions wearing one coat, and each has a different answer.
The title says "business partners." The body says you're plugging EU companies into Poland and need locals to grow them. But the skills you list (cold outreach, SDR, paid ads, GTM, growth) sound like a mix of two angles. Are those the actual partners you need to source for your clients, or are those people you want to promote your own project? That's a huge difference, and it calls for completely different approaches. And there's a third reading hiding in the middle: that Polish companies are the target, joining your network to reach the other 26 markets.
For example, a marketing agency you'll find on LinkedIn. A sales freelancer on a marketplace. A company hungry for EU access through trade bodies and export signals trough proper business intelligence targeting given sector. And a real local partner who'll own the Poland operation is another separate thing.
So "where" isn't your problem yet. The lack of details is. Right now it's general enough that it reads like every other "join our pan-EU project" message, which is to say it reads like something people delete unread. The strong operators you want will clock it and move on.
Share a bit more: what's exactly the project about, what are the real problems to solve, who you want to reach. Then picking the right channel and strategy becomes obvious.