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Numidian light auxuiliar cavalry can still be seen in Column of Trajanus (100 years later than Caesar), so they surely were in late republican roman armies.
Spanish cavalry is also a good choice, spain was almost conquered (except the celtic north) but was no much romanised so far in I C. b.C. In battles between Caesar and Pompeius in SPain , as Munda, there were a lot of native troops.
You can use also GERMANIC cavalry, (Caesar write about german auxiliary cavalry in De Bello Gallico), you can take some from Strelets of convert any infantry fromn caesar miniatures or Hat
I suggest you ase well using the Hat auxiliary and praetorian roman cavalries, maybe with some head swap from republican sets. Even the strelets coming mounted pratorians, with republican shields can be interesting.
Of ocurse, gallic would be also used if you do not mind using gauls against gauls (as it was the real History)
Do not forget late greek-macedonian or successor cavalry , they would also be seen in Middle-East in campaigns of Antony or Pompeius.