To research travel documentation and visa requirements I always start with two sources:
- the IATA Travel Centre, which provides personalised passport, visa and health requirements advice based on your personal details and itinerary.
- the website of the airline(s) which offer suitable route(s) according to your budget, departure airport etc etc. For example, on British Airways there’s a Passports,visas and API section
If you’re looking at self-transfer transit options, remember that typically you may require documents to enter the transit country, not just transit.