There are two ways to get conquistadors. You get whichever ones your country historically had, and you can sometimes get one by random event.
In Europa Universalis/DB/Leaders there's a file, historicalleaders.xls, that lists all your leaders, including when they arrive and when they die - a 'C' in the thrid column means that the leader is a conquistador. Note that not every country gets conquistadors; Austria never gets any, for instance.