The new pupptes war troopers ought to fit the bill perfectly.
I think they're resin but there you go:
Mmmm, now those are good looking and resin's fine, but I am a little gun shy with Puppet Wars. I bought some of their "not necron" robots a couple of years back and the casting was very hit and miss. Some items were perfect while others has lots of bubbles and two had nasty mold offsets. I'd need to check around to see if their quality has improved since then.
May I ask why you prefer them to be plastic?
You could chalk some of it up to my own internal neurosis, but I am running up against some practical considerations that make plastic more preferable to metal. First is durability of paint. I usually put at least an hour or more into painting a figure and I get a little obsessive about keeping it looking good. For metal, if the figures bang around too much, even from normal play, the paint can easily come off from chipping or wear. With plastic on the other hand, even an accidental trip from the table to the floor doesn't seem to harm paint jobs.
I also have a storage problem. My wife is kind enough to let my hobby fill up a 6' x 8' storage room in our house, but that's the limit. This summer I passed the 300 mark on painted figures, with maybe half being metal. To keep the paint jobs in good shape, metal figures require foam, which is pretty space inefficient. With more durable plastic, I can just rank them up in a sliding drawer with no worries. By way of comparison, my 50 plus Dust Allies figures take up one 14" square drawer, where my old Necrons take up almost four times that amount of space in foam trays.
If I find a good looking figure in metal I'll still pick it up, but nowadays that's the exception not the rule for me.