You are responsable to purchase your own airlines tickets. You can use a travel agent, or do it on your own using a travel website such at Vayama, Trip Advisor, Kayak etc. We purchased our tickets through American Airlines. Airlines that fly into Asuncion include American, LAN, TAM, Copa etc.
Airlines tickets need to be bought before applying for a visa. They need your itinerary as proof that you will be leaving the country. It takes about two weeks to get the visa so plan ahead.
Please let me know of your your flight details as soon as possible.