(Arbitrage) Upwork freelancer needing to source UGC content for client $5000 budget

Hey reddit family, I'm a freelancer on upwork. I have a client who asked me to source UGC content for their brand with a budget of $5000. 

1. How do I source content on marketplaces like billo without them knowing? (Arbitrage)

2)How do I get the budget from my client and pay on marketplaces without them knowing?

Looking forward to hearing your recommendations! :)