Post-Marian? Depends on where the fighting is. In Iberia basically every province yields Iberian Cav auxillaries at factional MIC 4 I believe, Gallic provinces yield Gallic auxillaries at factional MIC 3 and, if you've built up, most yield Birhentin (Gallic Noble Cav) at regional MIC 5 (also Bonnonia), the Thraikian provinces yield Thraikian auxillaries at factional MIC 3, and Germanic provinces yield Germanic auxillaries at factional MIC 3 or 4. Almost all the Western and Eastern Greek provinces (not the ones that would be classified Eastern) will have Hippeis at regional MIC 3. In the Eastern ones (Assyria/Babylonia eastwards) you get a flavor choice of skirm cav (Armenian, median, etc) at regional MIC 1 and possibly Median Medium Cav at regional MIC 3 (not sure). In any steppe-influenced provinces (Kallatis/northern Pontus Euxines coastal provinces + the steppes themselves) and in certain areas of Pahlava-influenced areas you can get lovely HAs at regional MICS 1 and 2 (maybe more for Dahae HAs). Basically all of Western Africa yields the Numidian Skirm Cav at regional MIC 2 (the best skrim cav in the game in my biased opinion), plus elephants west of Carthage.
The only real heavy cav you can get as Romans are the Birhentin or pre-Marian Equites Extraordinarii. But then, you're the Romani, surely your infantry is up to the task!