You have a few choices if you need to book a subsequent flight in order to fulfill entry criteria or apply for a visa.

Air Travel Representatives: To look for and reserve flights, you can use well-known travel websites like Expedia, Kayak, or Skyscanner. These services let you filter results according to your tastes and give a large selection of flight possibilities.

Straightforwardly with Airlines: You can directly make arrangements for future travel by visiting airline websites or contacting customer care. Flexible booking options, such as the opportunity to secure a seat without having to pay the full amount at once, are offered by several airlines. 

Services for Booking Temporary Flights: A few services arrange reservations or itineraries for short visits that are mostly intended to obtain a visa.

Although the reservation these providers make looks like a legitimate ticket, it isn't.

Travel Representatives: travel agents from airlines or areas could be able to help with additional travel arrangements. They can assist you with booking and in selecting flights according to your requirements.