It's not a field in the <Civilizations> table itself; the database table to do this is <Civilization_BuildingClassOverrides>, and Firaxis in the main game files put it in the same file as the civilization definitions themselves. So, for the bazaar and krepost, you can find them being mapped to Arabia/Russia respectively in Assets\Gameplay\XML\Civilizations\CIV5Civilizations.xml:
Code:
<Civilization_BuildingClassOverrides>
...
<Row>
<CivilizationType>CIVILIZATION_RUSSIA</CivilizationType>
<BuildingClassType>BUILDINGCLASS_BARRACKS</BuildingClassType>
<BuildingType>BUILDING_KREPOST</BuildingType>
</Row>
...
<Row>
<CivilizationType>CIVILIZATION_ARABIA</CivilizationType>
<BuildingClassType>BUILDINGCLASS_MARKET</BuildingClassType>
<BuildingType>BUILDING_BAZAAR</BuildingType>
</Row>
...
</Civilization_BuildingClassOverrides>
And for Assyria, in Assets\DLC\Expansion2\Gameplay\XML\Civilizations\CIV5Civilizations_Expansion2.xml:
Code:
<Civilization_BuildingClassOverrides>
<Row>
<CivilizationType>CIVILIZATION_ASSYRIA</CivilizationType>
<BuildingClassType>BUILDINGCLASS_LIBRARY</BuildingClassType>
<BuildingType>BUILDING_ROYAL_LIBRARY</BuildingType>
</Row>
...
</Civilization_BuildingClassOverrides>
[/code]
The former (vanilla) XML file also contains the table schema: