Buy mercenaries from the ransom brokers in the cities (if you have a gold bar and some surplus money). Be careful though high tier troops cost a lot of maintainance so even with 50000 you may accumulate a deficit pretty fast unless you hold a couple of towns/cities/castles as well. The best money comes from prisoners but if you're not at war with anyone you'll need a steady income unless you don't want to chase heretics etc. all the time. I usually get my best troops from liberating prisoners anyway. I don't buy mercenaries unless I want to garrison a newly conquered fief real fast.

The best way to build an elite army is to build churches in your Fiefs and build a training grounds in a city or castle to train nobles. Nobles level up quickly and can be upgraded to knights once maxed out. (If you build a knight order in your city you will also get free troops every week. 1-3 knights/squires)