Is it possible to walk into a travel agency in any large town, Mashad, in our instance, and buy two air tickets one day before the flight? Reserving from abroad is close to impossible, I tried to book with a UK agency earlier suggested on this post, but the request could not be finalized, so the only two options that are left is either get the tickets once we get to Mashad, or book them through uppersia.com in Iran. Any advice would be of great help for us.
It depends on the cities you want to book the flight for and what time of the your or days of the week you want to get the flight. Domestic flights in Iran are usually fully booked from weeks in advance specially in the busy routes such as Tehran-Shiraz or Isfahan-Shiraz , so it is suggested to book it as far in advance as possible.
You may try the online service of booking domestic flight details in Iran with Uppersia here.