a boss sustainer doesnt sustain notes, its a compressor, it compressed, kill dynamics in return the pathetic milisecond of sustain that doesnt help in anything at all...
fernandes sustainer does the job of sustaining till kingdom come, but not the kind that moore gets, in p.walkway
The sustain you migh have in mind, comes from couple of factors, amp, pickup, volume of playing, setup of guitar, guitar type etc
No particular gear will get long sustain, without adding other characteristic that you might not want it to be inside...
reverb adds sustain too, to the point of the note played will never fade away, but in return, it makes you sound like playing in a canyon
muff type effect has long sustain, in return, the gain within
certain delay pedal has a hold function which allows user to step on it and let the note goes on and on till you release your foot, but in return that last note of sustain doesnt sound natural at all
compressor give the effect of like having sustain, but it aint gonna add sustain that goes on without the effect of killing dynamics
imho