In the top left of the Army Management screen we have the troops you are using in your current army when attacking:
Here you can select to show only units of a specific type. From left to right these are: All, Fast, Heavy, Light, Artillery and Ranged.
These two drop down menus allow you to further filter troops by All units, Unattached units, Attached units and by era. However you will need Unattached units to be shown by selecting this in settings.