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.
Once you’ve narrowed down your options, check the official immigration website(s) of the relevant country(ies) too. Rules can change at short notice so if you’re booking well in advance I recommend you review them periodically in the run up to your departure date.