Search English-speaking customers without upwork?

That works for me:
1) Guest posts in the English-speaking blogs of experts in their chosen field (this by the way, is absolutely free and you can do - the main thing is to get in touch);
2) FaceBook Targeted ads (and landing on the target FaceBook page);
3) Thematic forum (I put a forum on 3rd place, because it works worse than 2 previous);
4) Your website/blog/YouTube channel in English. The most time-consuming, and the lowest ROI in the short term, but it is a long-term contribution.

On LinkedIn is fine to complete a profile and be friends with all colleagues. I write recruiters themselves quite regularly, although I am not in search of work.
On StackOverflow there is a section Jobs. There are interesting options.

https://angel.co/jobs - work in startups