I just lost the gas cap for my Honda Civic (1999).  I'd like to order a new one 
online.  If possible, I'd like it to be an authentic Honda gas cap.  Where can 
I get one?

You can purchase a gas cap for your Honda at www.carparts.com. The average cost 
for this item is about $5.00.

On the site you will need to enter in the specific model, year and type (I.e. 
LX, DX etc) for your vehicle and then you can search on "gas cap" and purchase 
the part. This item ships in 7-10 days. 

The website is: 
http://www.carparts.com/

I've found that most gas caps are universally interchangeable. If you go to a 
gas station and tell them that you drove off with your gas cap on top of your 
car they'll most likely pull a box full of them from under the counter and ask 
you if one looks familiar. Make sure you don't take one with a lock on it!

I really, really don't think there would be a difference in gas caps when 
comparing a 1999 Honda Civic LX with a 1999 Honda Civic DX...

Although I still think you should get the authentic gas cap. I bought a generic 
one and it failed the smog test. Yes, they test the gas cap when they do a smog 
test.

most Honda and Acura cars use a non-vented gas cap.   DO NOT get the
universal gas cap.   Running generic parts on your Honda / Acura is
like  one-size-fits-all pants.   they seldom work right.   I got
generic cap and rotor for my Acura once from AutoZone type place.  
Had a slight miss at idle. just a little hiccup once or twice at a
stoplight.    took it off, spend the extra money and got the good
parts from theealer.   178,000 miles on it today. still looks and runs
like new.  you can get GENUINE Honda parts discounted and sent to you
from this place http://www.ahmotor.com/      Good luck.