Chirundu is in Zambia.
From Lusaka either take a long distance coach in direction to Harare and ask the driver to dropp you off at Chirundu (Zambia!). These will leave at the Intercity Bus Station. Minibuses leave from the City Market Bus (Lumbumba) station, located west of Cairo Road.
Minibuses leave from Chirundu for Lusaka and other Destinations. As always, it's advisable to be early.
By Taxi. You can take a Taxi to Lusaka, if you are willing to haggle you can go there for 500ZKW. A reliable driver is Dunkin (0976621615)