<ComboBox SelectedItem="{Binding Path=PartyId}" ItemsSource="{Binding Path = PartyCollection}" ></ComboBox>
This is will not as you haven't specified the Source for the ItemsSource. You can ignore Source only if the DataGrid's ItemsSource (i.e. Collection of Order) has PartyCollection as member.
I suggestion you to create a Property
PartyCollection in the UserControl or Window where you placed the DataGrid
Then bind like,
<ComboBox SelectedItem="{Binding Path=PartyId}" ItemsSource="{Binding Path = PartyCollection, RelativeSource={RelativeSource AncestorType={x:Type loc:YourControlName}}}" ></ComboBox>
Here "loc" is the namespace of the UserControl or Window. "YourControlName" is the class name.
Happy coding.
Mark it as answer if it helps you.