There are 2 particularly good places I have found for this so far (for empire players at least, can't speak for republic)...
1st is northern Ilum, west of the jedi camp (heroic area) where the defending the deliveries quest is.
There is 1 elite droid in just about every group of mobs and a couple on their own. When scavenging their corpse they always give either zal alloy or durasteel
they should be easily solo-able for a LVL 50 (haven't tried with a lower level) some nodes also spawn in this are for added bonus! :D
2nd is the central area of the first map zone on Corellia there is an area (slightly on 2 levels) with quite a few large, elite, "spider" style droids
(i think they are called armoury droids or something like that, but don't quote me) this area is not quite as good as the other as in addition to the zal alloy and durasteel
they also drop neutronium and amorphous carbonite however the frequency of the zal and dura is still quite high.
this area is particulary good if you are having difficulty soloing the other droids or are lower lvl. There is also an area near here outside a story instance with
some elite turrets and elite probe droids but I couldn't find the place again when I tried to go back lol these also drop the zal, dura neutro and amo carb.
On Corellia, the XR-32 Hunter Droids (aka "HK") are made of Zal Alloy;)
Update: I just did a little experiment on Corellia running around the Blastfield Shipyards. The sample size is small so take it as you will. I killed 15 each of Devastators and XR-32 Hunters.
Devastator: 6/15 dropped zal
XR-32: 9/15 dropped zal
So it's roughly 50% chance for Zal on any >= elite droid. Also in the area I got a number of "Durasteel Shipwreck" nodes which gave 3 durasteel + 2-4 zal. I haven't tried Ilum, but Corellia isn't bad for acquiring Zal Alloy.