How can a foreign national apply for work in the publishing industry in the US/UK?

I’m a recently graduated South African citizen hoping to find work in the US or the UK, specifically in the publishing industry. I’ve applied for a couple of vacancies found on job listing websites, but often there seems to be requirements for the applicant to be a US/UK citizen, respectively. I’m wondering if anyone knows which portals I could go through to seek entry-level employment? Thus far I haven’t found any agencies that deal with publishing jobs.