Do you just want to pick some up and keep them captive or are you planning on breeding for re-introduction purposes???
The latter is a noble idea but not a very practical or good one. It should not be done.
If you just want to get some, then check your local laws, if they allow it then it´s legal and you can do it.
However if the population is crashing i can only think that depriving the population of even more adult newts is not going to help at all. Bare in mind that you are not only taking the adult newts but also all their potential offspring(which can make an impact on a given population).
If you want to help the species there are a few things you can do that do not require undermining the troubled population. If you have the possibility, creating ponds, or even just hibernaculums close to where the population exists will probably be of great help.
Capturing a few animals and bringing them into captivity is not going to save the species. The species is fairly well stablished in captivity(though we need a lot more captive breeding). If you want to contribute, then acquiring some CB stock and breeding it yourself is a decent way of doing so. Taking animals from the wild, i insist, is hardly of help...
I hope you see my point...but anyway if you decide to take them(provided the law allows you to), check this link, which has all the necessary info:
http://www.caudata.org/cc/species/Notophthalmus/N_viridescens.shtml
I should also say that wild caught animals don´t always respond well to captivity...even if they go directly from the wild to the tank(instead of suffering the horrible journey that pet-shop animals do).