I'd like to replace the trigger on my Bushmaster lower.  I may get a target-style upper for it, but I don't want to in any way compromise the reliability or ruggedness of the lower.  I will be keeping the lower and regular upper as my SHTF rifle.  I just want a smooth, crisp single-stage trigger.

So just what is a "National Match" trigger for the AR?  I've seen them advertised, but I've never seen one or tried it.  Is it as reliable as the original?  Is it a good trigger?

If I choose to go this route, is there a way to get the necessary replacement parts without getting a whole lower kit?  What parts make it "Match"?

Is this a stupid question, and should I just go for an aftermarket trigger?

Most of the "match" trigger/hammers recommend they not be used for LE or combat use, so if you only have one lower you might not want to replace it. I have an adjustable, single stage, JP speed hammer/trigger setup in a Colt using the mil spec springs and it works quite well, very smooth and crisp. Has reliably fired in excess of 5000 rounds. I just recently put their 4.5lb spring set on a lower with the mil spec hammer/trigger. Have about 500 rounds on that with no malfunctions and it actually feels pretty good.

I use Jewel triggers in my High Power competition rifles but I would definitely NOT recommend one for your only lower if it's your SHTF or defense rifle.

Get a RRA two stage from John at White Oak Precision.  A lot cheaper than a Jewel, and you don't have to worry about set screws coming loose, so it would work fine for defensive use, although you may not want a 4.5lb pull for defense.  I have quite a few rounds through my tuned RRA and it is awesome!  Definately worth the money for a match trigger.
"If I choose to go this route, is there a way to get the necessary replacement parts without getting a whole lower kit?"
-Yes, all you recieve is the trigger itself and the hammer, with two pins and springs.  That's with the RRA trigger anyway.
If you do a search for 'trigger' you will find LOADs of info.
-Marksman