If you are trying to sell your music, you need to have acquired your beats and instrumentals legally, otherwise you could open yourself up to potential lawsuits. Legal methods of obtaining beats and instrumentals include buying or leasing beats or finding free downloads that the producer placed online for free and fair use by other musicians.
There are a ton of resources on YouTube for you to find the beats and instrumentals that you need to create your music. Just because the beats are free, that does not mean that they are subpar. You just need to look around to find the perfect ones for the music that you are trying to make. There are a lot of choices to get through but with enough patience, you can find the perfect options.