Could well be a sample library (from a real bass), one example is Scarbee Jay. But at least Madeon said in a Reddit AMA that he had a friend play some bass slaps and slides and whatnot for some tracks. I would check these out for freebies: https://www.conversesamplelibrary.com/pack/la-bass-connection-funk-jams

I would just drag the samples on a track and chop, stretch, pitch and arrange the clips to fit the track. That's probably the easiest way when you're working with samples of multiple notes. You're right that you can't pitch shift them very much without losing quality/realism, so the best you can do is select samples that are close to the intended pitch. Of course it helps if you can choose your track's key to fit that of the samples.