i don't have the patience to work with samplers like i can synths so i can't help with the technicalities but i do think i have a helpful perspective to share that might answer your question since it wasn't a technical question anyway. but i need to toss some hip-hop history out there for perspective:
you have to master the ancient art of crate-digging, invented by dj kool herc and mastered by 80's hip-hop dj's
i grew up on 80's/90's hip-hop, my life used to revolve around it and my favorite dj/producers always sampled. most of em say they grew up listening to funk/jazz/etc records their parents would play so they were familiar with the genre's sound already. this would help them know more of what it is they wanted to start with by having a good idea of how varied the genre can be. then they'd go through their parents' vinyl collection or down to the record shop and play vinyls for hours until they found some that are good for sampling... or they'd just buy a few that had promising names or interesting album art if in a rush
then they'd play what they wanted to sample and throw the promising part in the sampler and go from there. you can imagine how many vinyls most of these guys had, practically a small library's worth after years of doing this.
as far as sourcing material outside of commercially released jazz/funk/soul... Nu:Tone uses a commercial library sample, commercial as in its aimed towards advertisements/commercials IIRC, he mentions the name in the video (youtube url is at the end of the post). also like spyre said you can dirty up and use your own samples. you can use a VST that samples the real instruments, there's a lot of em but some popular ones are omnisphere or kontakt and buy some funk, jazz, or specific instrument libraries for whatever you want. then write your own melodies and resample, maybe find a cassette tape recorder and transfer it from mp3 to cassette tape then back to add some real analog grit. it'll sound different from using a comparable sample off some commercially released material b/c sampling a real, live played instrument always has a better groove than using the sampler software IMO due etc different velocities, diff recording material, tons of diff details but it might get you close enough to where you're saitisfied and it'll be a nice 100% license-free legal approach. so that just goes to show you can not only get creative with what you do to the sample but you can get creative as far as where to find the samples too.
these days some things have changed but it's still the same basic routine. you listen to songs on youtube/mp3/etc for hours just like they did with the crate-digging. except now there's an advantage using tools like spotify, like riisu said, or if you start to notice multiple good releases from the same label then pull up their discography of releases and check em out. so today it's easier to find related material with all the websites that have the "you might also like..." recommmendations. it just takes getting more acquainted with the genre and its artists really. so instead of digging through crates for hours, now you spend hours digging through digital media. it's still the same practice, just with a software sampler substituting a hardware sampler (for most ppl these days anyway), with streaming/digital media selling websites replacing record shops and finally with digital media replacing vinyl.
I also started watching the Nu:Tone tutorial but didn't get round to finishing it. The way he uses samples might also be worth investigating.
Yeah the Nu:Tone tutorial is a great reference IMO although he does more work recreating the sample rather than working the actual sample itself. but the OP might find it really interesting because it's a good video for anyone interested in sampling. here's the Nu:Tone video
http://www.youtube.com/watch?v=TPPhWcoOSMY