Music producers get sounds from a variety of places, depending on their production style. There are two forms of sound a producer will use. They’ll either use pre-recorded sounds called samples, or they’ll use real and/or digital instruments, which can be played live.
For sample-based producers, Splice, Youtube, and various music forums (ex: reddit.com/r/drumkits) are all valuable resources for getting sounds.
Producers will often times chop up loops from these resources to give their songs more originality. There is no limit to what you can find: loops, midi, and vocals are all elements you can find online and flip into a unique song.
Here’s a song I made using mostly samples.
The vocals were pulled from Youtube, and the drums/FX were pulled from Splice/Forums.
Popular Sample Sources
Music producers who are more hands on can just use live instruments or VST (Virtual Studio Technology) instruments.
Live Instruments – Live instruments are things like keyboards, guitars, drums, and woodwind/brass. You’ll need a microphone to record them into your DAW (Digital Audio Workstation). You can find a variety of live instruments on Amazon or Guitar Center.
VST Instruments – Virtual instruments are similar to Live Instruments in that they can be played in real time, with the difference being the sounds are digitally produced. Synths like Massive and Serum are popular for their use in producing a wide array of sounds.