Where should we look for Python experts with AI / ML experience in a marketplace / online platform context?
Don't worry about country. As long as there is no communication differences between you expressing your idea and them executing it than you'll be golden.
You can use the classic places like bHance or Freelancer, but let me pitch a different idea.
Try to Google your 'project' or find something extremely similar to what you're looking to design. Find the creator and look to contact them. This way you'll know that they've developed something similar (within reason) and that they are up to the task.
Go for the fisherman, not the bait.
1. The US and Europe are probably expensive.
2. Options for outsourcing would be Eastern Europe, India and Vietnam.
3. Don't know much about Eastern Europe, but I do know financial product platform work being outsourced from Estonia to Vietnam. India has been tougher of late to get good talent, because most of the good talent gets absorbed by MNCs and startups. Right now, Vietnam is a good place because its in the early stages of the development of its software development industry.
4. For the run of the mill machine learning etc. based off some AI-as-a-service, it is possible to find various providers (I know companies using AI to identify consumer metrics that consumers are viewing on various car info sites etc.). For higher level algorithms, it just take some creativity to find researchers at the various institutes of higher learning and get them to consult.
India, Thailand, Kazakhstan, Russian Federation. good quality and not expensive.
Germany is really bad in outsourcing online platforms and their web design - 100€/hr and still full of bugs