Your saddle is one part of the bike that I don’t recommend buying online unless you know exactly what you’re looking for. Everyone’s bum is different, and what’s comfortable for one can be dreadfully painful for another. Instead, I recommend that you spend a bit more money, and go to a local bike shop where they have loaner saddles that you can try out for a few weeks before committing to buying one. But if you already have a favourite saddle, or if you have decided to try your luck online, Bike Nashbar has a very large selection of name brand saddles at very competitive prices. They also have their own branded saddles at even lower prices. (I ride a $25 Nashbar saddle and it’s perfect for me, but you may not be as lucky.) Nashbar has 20%-off sales almost weekly, and shipping within the USA is fairly cheap. If you live in Canada like myself, shipping is more expensive, so it’s best to wait for one of their many sales. Another online store to shop for is Performance Bicycle. Their website is very similar to Nashbar’s. Performance Bicycle doesn’t have its own brand (at least not that I know of, but I suspect that some of the brands that they sell are actually identical products to Nashbar’s). If you live near one of their retail locations in the US, you can have the saddle shipped to their stores for pick-up for free. Otherwise, shipping rates are comparable to Nashbar’s.
A lot of times seatposts and seatpost clamp are included in the frameset. But even if they’re not included, they’re not difficult to buy. Seat posts for carbon frame usually have diameters of 27.2mm or 31.6mm and lengths of 300mm, 350mm or 400mm. If you bought a seat post that is too long, you can always cut it down a bit. (Do it carefully though, and don’t cut too much.) Buy your carbon seatpost on eBay. Since a few months ago, the prices have been steadily dropping. Now you can get a carbon seatpost for about $20 USD. Pay attention to the weight of the seatpost though. Some have a large alloy mount for the saddle and that’s where you might find excess weight. A typical carbon seatpost weight about 200-250g. (Of course, you can always get an alloy seatpost if you think that there is too much carbon on your bike! Alloy isn’t that much cheaper though, but at 270-300g, they’re not that much heavier than carbon either.)
Now that you have a seat post, you have to secure it to the frame with a proper clamp. The clamp is usually alloy, and that’s fine. You won’t save much weight from using a carbon clamp anyway. Also, the difference between paying a few dollars or paying an arm and a leg for a seatpost clamp is at most about 5-10g. When buying, pay attention to the clamp size. In general, they’re either 31.8mm (for 27.2mm seatpost) or 34.9mm (for 31.6mm seatpost). The carbon frames that you buy online is almost guaranteed to fit either one of those two sizes. Again, eBay is your best place to get the clamp at the best price. You have a large variety to choose from, and they’re usually inexpensive. Expect to pay less than $5 USD. I included a recent example (from Nov 2014) on the right. Whether you want the clamp with quick release is a matter of personal preference. My feeling is that you shouldn’t get a quick release. If you ride a road bike, you’re unlikely to adjust your seatpost frequently (unless you lend your bike to other people often). Having a quick release also means that if you ever need to park your bike on the street (Gasp! Not recommended with your carbon masterpiece!) you’ll have to take your seatpost with you so that someone else can’t just walk off with your saddle.