They are sold at digikey
and also consider the MAX8212. Far more expensive than a MN1381 but its trigger voltage is configureable.
the only place i have seen that sells it is www.solarbotics.com but its pricey .
i found a list of substitutes somewhere, but couldn't find any of the alternatives either.
What is a 1381 and what are you using it for?
Why not use the FLED circuit then?
There, that's every option.
probably wont help much but i was looking for one today .... lucky me found one on the first board from my recycle collection ... first part i looked at too ... lol. So if you have any old electric stuff check them out you might find some.