I own a Stag Arms AR complete with a cheap looking plastic handguard.
What do I need to do to get rid of it and install one of those nice aluminum guards so I can attach accessories to it?

Step 1.: Sell another gun to finance the purchase of accessories

Couldn't resist!

You don't have to remove the handguard to add accessories. There are rails made that attach into the holes in the factory handguard.

If you choose to add full rails, I put a Daniel Defense Omega rail on mine. It's free float!
Your instruction manual should tell you how to remove the handguards. Just pull the ring to the rear of the rifle and lift the handguards toward the front sight.

I reckon you will struggle to find anything much available due to the present buying frenzy.

Google AR-15 free float handguard & you will have a multitude of different models to look at.
Most use a modified barrel nut to mount them rigidly.
I can't recall the make of the one I have but fitting was easy enough.