Try Sonniss.com ProSoundFX.com and sounddogs.com for finding lots of small packs you can buy for pretty cheap most of the time.
The main guys would be Sound Ideas and Hollywood Edge, but aside from those too, there are other slightly less big names like Boom FX, Bluezone corporation, Blastwave FX, and more.
For totally free stuff, look up the "great big list of free sounds" from itch.io.
There is also Freesound.org, the free music archive, and some sites that sell things sometimes have free stuff too, like soundimage, audiomicro, blastwave, airborn sound, 1SoundFX, imphenzia, and more.
This... -- Is CNN'.
Well Ted, it sure looks like there's been uh, quite a bit of violence around here
"aaoh, that violence was terrible'!"
Yeah it was, pretty bad.