The XB241H or the XL2420G are two of the only 1080p, 144hz, G-Sync 24" monitors around.
Both are quite overpriced though, I wouldn't recommend picking up a G-Sync monitor unless you're going 1440p, where it becomes much more relevant.
if you have something around 970 level performance you should push over 60fps at 1080p most of the time anyway, meaning you don't really need G-Sync.
If you DON'T have 970 level performance however, you are much better to spend your money on a GPU upgrade as a pose to a G-Sync monitor.
I recommend going for the BenQ XL2411Z or the Asus VG248QE which is listed above. Go for whichever is cheaper, as they both use the same panel, and are great value for money.