Caesar had to utilize the resources which were available to him at the time, and that meant using cavalry from his local area. He did not have the mandate to recruit cavalry from Spain, as that was someone else's area of imperium. In 1.15 of Gallic Wars Caesar says that his initial cavalry muster was from all parts of the "Province" (he means the two Roman Gauls and Provence) and from the Aedui, who were a powerful Gallic tribe just to the north of the Province who were Friends and Allies of the Roman People. It was the protection of the Aedui from the marauding Helvetii which ostensibly got the Romans involved in the Gallic Wars in the first place. Initially, he got "enough" cavalry from these places (though you can never really have "enough" cavalry). Later, he was forced to draw on more shaky Gallic "allies" as the level of his involvement and therefore the size of his forces grew.

For why there was no Roman cavalry with Caesar, see this thread.