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.