Often these parts are not needed to be purchased separately: Many upper receivers come with a forward assist button and dust cover and many rails come with a barrel nut. As with most lower parts, just get the cheapest parts you can find, which again for me is usually DPMS from Brownells.
Some of the many engraved dust/ejection port covers on offer.
However, the dust cover/ejection door provides an opportunity to add a little customization. There is enough real estate on that guy to show some personality on your weapon if you should so choose.
Spikes, Brownells, Rainier Arms and several other places stock them, though I suspect most come from Harford Engraving. For a while Noveske carried a replica of the dust cover from Team America. "Surprise cockfag!" for those who don't remember. Sadly they stopped some time ago, before I could get one. However if you really want your own unique cover, just take a blank one to any local engraver with what you want.
I do have to make one word of caution: Several places sell covers that feature certain military units. Seal Team 6 is one example, though I have seen others. While some people may feel they are supporting their favorite unit by getting such a cover (or for that matter other part, T-Shirt etc), most of the guys that served in the unit will not see it that way. It is less true of units that have no selection process, but still likely that you will piss off the guys you might be trying to respect.