General synonyms:
* Whipping boy
* Fall guy
* Target
* Sacrificial lamb
* Sucker
* Patsy
More specific synonyms:
* Blameless victim (emphasizes the innocence of the scapegoat)
* Proxy (suggests the scapegoat is used as a substitute for someone else)
* Scapehorse (a humorous synonym)
The best synonym to use will depend on the specific situation and the nuance you want to convey.