One of my favorite shops is a Specialized dealer, but they can be ordered online. I did a quick google search and found this place (I've never ordered from this shop, so I don't know how they are as a business). They are listed on ebay, but the prices are actually higher than in the shop.

Be aware that there is a mountain bike version and the Rib Cage Pro for road bikes is the good one. I've read some negative reviews of this cage, but I don't know what people are talking about. My experience is that they hold bottles better than other metal and plastic cages I've used, pulling large bottles out from the side and jamming them back in is better than other designs. Durability is decent and certainly better than cheap metal cages that break within a season.

EDIT: just found out that the link I provided says Specialized products can only be bought in the store.