If I’m not concerned about commercial use, I sample from my record collection (meaning, iTunes downloads, CDs, etc.) I may or may not have used a few of those in commercial situations too, when it was something obscure, fragmentary, or heavily processed. Sometimes I get access to audio files from friends’ projects, and sample those with permission. I’ve made a few nice field recordings that I go back to routinely. A friend of mine recorded a whole collection of extended violin technique for an NYU composition class, and I asked the studio tech to dump the whole thing on a thumb drive. Apple loops have amazing sound quality, and while they tend to be corny, you can always process and edit them. Freesound is great if you stick to the WAV files. I’ve bought a few royalty-free compilations of different things that are hard for me to obtain otherwise, like vinyl scratching. If I got heavier into commercial producing, I’d invest in a bunch more of them.