Visa requirement questions

by sarah
(Houston Texas)

My apologies in advance if these have already been answered -
1. Can US citizens process their visa upon arriving in La Paz? I am getting mixed responses.
2. What is accepted regarding proof of exit, or do they require that before issuing the visa? Do they require a flight ticket or would a bus ticket or simple itinerary suffice? If a bus ticket, can you recommend a travel agency that would book the ticket online so I can have that proof?
Many thanks in advance for any help.