Firstly, snapshots are called baseline sets in the RTC Plain Java API. Knowing this, you can get the snaptshots via IWorkspaceManager.findBaselineSets():

ITeamRepository repository = ...;
IWorkspaceManager workspaceManager = SCMPlatform.getWorkspaceManager(repository);

Just hand in your given stream as the workspaceHandle to the following method:

@SuppressWarnings("unchecked")
private List<IBaselineSetHandle> getBaselineSetHandles(IWorkspaceHandle workspaceHandle)
                                                throws TeamRepositoryException {
      IBaselineSetSearchCriteria searchCriteria = IBaselineSetSearchCriteria.FACTORY.newInstance();           
      searchCriteria.setOwnerWorkspaceOptional(workspaceHandle);

      return workspaceManager.findBaselineSets(searchCriteria, Integer.MAX_VALUE, monitor);
}

Then you can loop through the returned List<IBaselineSetHandle> and get each BaseLineSet (== snapshot) with the following method:

private IBaselineSet getBaselineSet(IBaselineSetHandle baselineSetHandle)
                                                throws TeamRepositoryException {
        return (IBaselineSet) repository.itemManager().fetchCompleteItem(baselineSetHandle, IItemManager.DEFAULT, monitor);
}

Hopefully, that is of help.