[jerryb]

Where can I buy 870 chokes online?

 

[a1huntingsupply]

Are you looking for Remington brand choke tubes only?

Carlsons and Hastings make high quality choke tubes for Remington shotguns.

 

[A5Mag12]

If you are not in a hurry check ebay. There are regular sellers on there that if you wait until you get them for the opening price you can get a heck of a deal.

Otherwise this is as good as any.
http://www.ableammo.com/catalog/default.php

I wouldn't go out and buy high dollar aftermarket chokes until you are dang sure you need one. RemChokes are probably the best patterning factory chokes out there.