There is a direct bus daily (may be except Sunday (?)) from Harbin to Vladivostok which departs from the bus station near the main railway station (500Y). But very often it is cancelled. Instead of it you can take bus to Suifenhe or Mudanjiang. From Mudanjiang bus station there are two buses (about 6 and 9 am (I am not sure)) to Ussuriysk. They stop in Suifenhe at international bus station at about 8-9 am and 1 pm.
Buses from Harbin arrive at railway station in Mudanjang and at local bus station in Suifenhe.
There are frequent buses from Mudanjiang (from railway station) to Suifenhe and from Ussuriysk to Vladivostok.

thank you for your help!