Most samples I use are either drums & one shots off the MPC or being used in sound design on the rackmount sampler as waveforms. I use the rackmount to trigger entire stems too; I have gotten into the habit of taking synths I've sequenced with MIDI on the MPC and sampling the audio into my other sampler as soon as possible. Part of this is to approximate the convenience & flexibility of working with audio that you get in software these days like Ableton, etc., part of it is to not let ideas linger by committing to them upfront and making it easy to get in that habit. Committing to audio helps lock in the composition too; when I've gone from MIDI to this chunk of audio, I feel like I have an actual building block I can put now into place and have interact with other pieces.
So I'm more or less sampling my own stuff, but only for the sake of having it in audio form. Sometimes I manipulate or work with the audio more in stem form, but I feel you really shouldn't be committing your 4/8/16 bar idea to audio stems yet if you're going to **** with them more...but sometimes that's what you have to do to get a desired effect. I used to bounce drums, bass, rhythm down to one stem sometimes to play other things over and you can get interesting results playing these stems at different pitches/tempos and different amounts of timestretch applied, but I don't find myself doing this much more and it's probably because I don't have the same mixing/recording limitations.
When it comes to sampling, every once and awhile I come across a cool drum loop or bassline in a published artist's track...but I'm usually just stealing a bass note here and there, the typical sustained note off the string section, random synth bleep I can trim & loop and use for an oscillator, etc. I find a sound, put it in the rackmount, run it through effects & modulations, resample, rinse, repeat. Resample, resample, resample. To me, sampling is just a really diverse synthesis & sound sculpting technique.
Song I'm working on today is a bassline on my MS20 being sequenced by the MPC. MPC is also playing the drums I sampled into it long ago. I came up with an fx type sound on one of my synths, so sampled that into the MPC so I can now use that monotimbral synth again. The rackmount sampler is being triggered by the MPC. On the rackmount, it's mostly vocal samples. I took vocals from Grace Slick (Jefferson Airplane) and looped them so it's kind of a soft "ahhh" chant. I took vocals from Whitney Houston and chopped it down to a small sound. At some point I sampled some sort of bells/windchime. I've layered all this and played it chromatically to get the brunt of the track.
So while I do sample...I mostly sample for the sake of creating new waveforms/textures/sounds more than I do it for compositional reasons.
MPC 2000XL, Korg EX-8000 & MS-20 Mini, Behringer Neutron & Crave, Arturia Microbrute, E-mu E6400 Ultra, circuit bent Roland TR 505, Mackie 1604 VLZ3, Electrix Filter Factory, delays & compressors