If you like your saddle further forward and you deal well with the extra weight on your arms then go for it. I'm not certain what the offset is on my Tarmac, but it's not much if any. And the saddle clamp is roughly in the middle of the rails. If your clamp is on the extreme of the rail adjustment, I'd get a seat post that puts the clamp back in the middle. Then I'd have more adjustment options open for times I might want them.
I'd buy from any online site that's been in business for a time and you might even find that your LBS sells them just as inexpensive. But that does vary. Some things they'll sell more at cost others they'll have to have their mark up on.
Universal Cycles and
Jensen USA are two of the online places I use quite a bit along with
Tree Fort Bikes.
But I've also had no issue with others I've used when they've had the better price + shipping. But do beware of the too-good-to-be-true prices you might find.