It depends what search terms you're using. The MUL search is specific, so if you say (for example) "Marik infantry", you won't get anything because there's nothing with the exact term "marik infantry" in its name.
Two ways to find the standard infantry units:
1) Search for just "infantry". then go to the "Infantry" tab (eg. ignore vehicles and aerospace units with the word "infantry" in their name.
2) Use the Search criteria to pick a faction & availability era (eg. "Free Worlds League", "Clan Invasion"), then go to the Infantry tab & see what they had at the time (note: includes battle armour, so some common sense required.)
Hope that helps! There's tons of standard units, with names like "Foot ballistic rifle infantry", "Ad hoc Clan foot infantry", and the like.