Try having a sample of a top kick that carries the primary attack sound of your 808 (a click or an acoustic kick's thud or whatever you feel like), then like Heymac said, take a synth and output a pure sine wave - treat that like your 808. For the more rich sounding 808s, throw a saturation plugin on top to add in some extra harmonics.
An 808 kick drum is actually something very simple to create with a single oscillator - it's a sine wave with a really quick pitch fall at the very beginning. Using something like Operator in ableton, you can make a key-tracked 808 in two easy steps:
- Take a sine wave and put a volume envelope on it with no attack, about 1 second of decay, no sustain or 100% sustain (try both!), and about 100 or so milliseconds of release.
- Put a pitch bend envelope on that oscillator with no attack time, about 150-200 ms of decay time, no sustain time, and the release is kinda irrelevant. 2-4 octaves of difference between the start of the note and the end of the envelop should be a good enough range.
Now just find the right note for where you want the kick to land, and voila! For more customization, set macro controls to adjust the attack and decay times of the two envelopes and the sustain level of the volume envelope, the amount the pitch envelop affects the sine wave, the distance between the start of the sound and the end, etc., etc. Save that as a preset in your library, and grab it whenever you want to
Plus all you need to do is disable the pitch envelope and set the sustain to 100% and you have a pure sine wave sub bass for any other kind of tune you make. It's not necessary but it usually considered a helpful pre-mixing practice to have a separate sub in your track and high pass anything that you don't want to have sub frequencies (so basically everything other than your kick and your sub bass) at a point just before you start hearing it actually affect the sound, usually somewhere between 100-350 Hz but you will know when the sound starts to hollow out just from listening.