bus is the best way to see peru in my opinion.
to answer your question though no i don't know of a company or website through which you can get routes and book online. when i was traveling in peru i just bought my tickets right before boarding the bus. the bus stations have tons of bus companies. things to consider when choosing: price?; seatbelts?; what time will i arrive? (apparently it's safest with regard to both crime and traffic collisions to try to only travel during daylight hours); are there TVs on board? (i got so sick of dubbed awful american movies that i wanted to scream); and meals on board? (only the nicest ones offer this). that's all i can think of now.
you can get the different companies' routes at the bus stations.
the only reason i can think of for making reservations ahead of time would be if you're on a tight schedule and might hit a holiday weekend.
good luck have fun and let me know if you have any other questions.