build one just like an Alembic.
I rep for a local builder here in my hometown who makes handmade guitars, and custom patented bass bridges. The bridges definately add sustain over ANY other bridge I have ever heard or that we have tested. They run about $120 a piece. Feel free to send me a PM if you are interested in more info
A nice solid bridge will add some sustain, as will a neck-through bass. On recordings, you might also be hearing the helpful effects of a compressor/sustainer. Sustain is an effect of several factors.
Boss makes a compressor/sustainer pedal, but most compressors I think would do the job. The reason why a compressor works is because it takes the quiet tail of the note and brings its level up higher so that it sounds like the note goes on for along time. If you take a compressor and set it with a low threshold, long release time and high ratio, it would make the note ring on a lot longer, but it might start to pump and breathe, making the sound maybe not quite what you wanted at first. If you do that, make the knee pretty soft to maintain a more natural sound. Of course, if you wanted to go electronic-less, heavy slides can help too.
JTC, I do know quite a few people. I work in the music industry for a living whether it is at my studio, working at someone else's studio, or mixing live. I work anywhere form 20 to 100 hours a week depending on how I feel and how busy things are. As a result, I meet LOTS of different people.
So between the neck joint type and the mass of the bridge, that should give you a couple of features to be on the lookout for
as you shop for your next bass.
There is a lot of lore around as to what makes for increased sustain - through neck vs. bolt on vs. set neck, heavier/denser bridge structures, brass nuts, carbon fiber necks, ways of winding strings, etc. ad nauseum. Most if not all the evidence is anecdotal.
Go to a music store, try some basses, and see what floats your boat.