The sound I've been getting all these years is from an early 70s MXR Phase 90, a little orange box that is still marketed, but with updated circuitry that is noticeably different to me and not as appealing. It's very unlikely you'll find a box just like what I have, as the circuitry got changed in 1976, and most models that were sold were the later kinds.
There are many effects devices nowadays that are versatile and can create phaser-type sounds. It's similar to "chorusing", but there is a warble whose speed you can control on the Phase 90, and I got used to a particular speed ("1:30" on the knob) which I always used.
Boss used to put out a device called the SE-70 (earlier the SE-50) effects processor, that had a setting called "vintage phaser" which with some tweaks was a very close match to the Phase 90 sound I like. If you happen to get hold of a device like that, check back with me and I'll give you the settings. There is probably a current version, but I don't know the model number.
You can always take a recording of the sound you want, to play for a sound guru who knows about sound processing devices, and he/she might be able to steer you toward a good device and appropriate settings. But it's hard to find the right guru who's not merely into selling you something that "can do everything". It would be great if they'd take the time to actually get it working the way you want it, on your instrument.
The phasing is more noticeable and attractive on the lower tones, so as you play, work the 4th string a lot, with slides. To hear it well, you'll need headphones, preferably the kind that block out some or most of the acoustic sound.
Best of luck. Let me know what you come up with.