We recommend you to check different options for flying from Lima to Iquitos. LAN Peru, StarPeru are good options, and you may book from the internet. Check them all to find the option which suits you best.
You may buy your ticket once you are in Peru (also on the Lima Airport) or through a travel agent in Peru. The costs of buying it on the internet are high (specially with LAN Peru: if booked outside Peru, prices are double), so if traveling on the tight budget, you might want to check other options.
We recommend you to bring some cash. There are ATMs in Iquitos, but if you have a limit on the daily withdrawal, it may sometimes give you a bit of trouble. If you decide to bring cash, bring the nicest, crispiest and newest bills – the standards for quality of dollar bills here are extremely exagerated. They won´t take a bill if it has a super tiny tear or if it´s old, or if it´s rugged… If they take it, they will take a comission. Traveler cheques are also an option but they charge comission.
We’d like to stress that you need to take care of your personal safety. Please don’t trust the people who might approach you and say they are our friends, asking you for money. If we coordinate a taxi to pick you up, then they will let you know who they are, and the only money they will ask for is for the taxi service.