Its not exactly a piece that sees much abuse, but yes, there are places that send out plastic pieces.  If it bothers you, pick up an aluminum guard the next time you place an order with one of the vendors.  I wouldn't make a special order just for that or you would pay more for shipping.

What brand is this on?

Quoted: I agree it will hold up to normal use, I would just prefer an aluminum one. I have several more lowers I want to build up but I dont think I'll want the GI pistol grip or hammer/trigger on any of them. If you or anyone could give me a couple of dealer names on this site who sell all the other detents, springs, pins, metal trigger guard etc. I'd appreciate it. It was Bushmaster, by the way. Great company in my experience. This is not meant to be a slam on them in any way.

You may want to consider buying a complete lower parts kit, and then buying the grip, hammer & trigger.  I've seen the individual pieces cost more than a complete Lower Parts Kit.  If you buy the entire thing, you can always sell off the hammer/trigger.  Another option would be to contact a dealer like www.rainierarms.com, and see if you could buy everything and get a discount for the pulled hammer/ trigger/ pistol grip.  If you are buying everything from one source, I think you will find it more likely that someome will be willing to work with you.