contact the owners of the rights to the song. they can cost alot, or sometimes free. it depends on a relationship. it's no different that artist using beats. the more famous the producer is, the more it will cost. if you use an agency, it will cost more.
for instance, I have a few friends who are amazing musicians, I have access to there entire catalog. I can sample everything from them. I consider them cowriters, so everything split. but no advance. but if i make money, they will. with them clearing a sample is easy.
but if i was trying to sample from a random record i found in a crate, that will be harder. i can try to use an outside agency, but they will cost. i can do it myself, but tracking down the owners can be hard. then trying to convince them to clear it might be harder and can cost.
but you wanted to sample without doing the research first. true you can hustle beats, but that's what it is a hustle. next time, if you want to sample, sample local musicians instead. people you can talk to.