How to find a spiritual master after Srila Prabhupada's departure
Srila Prabhupada inspired many, many people to give their whole life to the practice of Krishna consciousness. In his books, he repeatedly stresses the importance of finding a "bona fide spiritual master." This is an eternal principle. He intended the Krishna consciousness movement to continue for as long as possible. This means he intended it to be a genuine, living spiritual tradition.
Those of us who became interested in Krishna consciousness at this time in history—just after Srila Prabhupada's departure from the mortal world—have the benefit of being able to read all his books, read all his collected letters, hear all his recorded lectures, and meet many of his disciples, all of whom have been practicing Krishna consciousness since 1977 at the very least.
Our advice is to avidly read Srila Prabhupada's books. He wanted all his followers to become conversant with the philosophy of Krishna consciousness, and his books are intended as study guides for any aspiring devotee of Krishna. By thoroughly going through his Bhagavad-gita, Srimad-Bhagavatam, and Chaitanya Charitamrita, anyone can get a complete picture of what Krishna consciousness is.
We also recommend that you hear from as many senior disciples of Srila Prabhupada as you can. If you've done your homework, you'll be better able to discern who has done the same. You may discover that some devotees seem to understand Srila Prabhupada's teachings more deeply and comprehensively. Srila Prabhupada's books are our textbooks for spiritual advancement, so in choosing a spiritual master it makes sense to try to find someone who understands Srila Prabhupada's books well enough to teach them.
Find someone who you feel best represents Srila Prabhupada. Hear from them as much as possible, and develop a relationship with them based on service. This is the traditional way to approach a guru, which Krishna Himself recommends in the Bhagavad-gita. Over time, Krishna will make it clear to you who inspires you the most in Krishna consciousness. It is said that by the mercy of Krishna, one gets a spiritual master, and by the mercy of the spiritual master, one gets Krishna.
One recommended site for hearing classes by senior devotees is Iskcon Desire Tree. There are hundreds and hundreds of lectures to choose from.