MiniDisplay Port to HDMI cable. (Unless the monitor only has DVI, then DisplayPort to DVI).
Apple doesn't have a genuine cable, but there are others that do. Belkin has exclusive products on the Apple Store site, but Amazon has a bunch, too.
My advise is to read the reviews starting the with 1 stars and moving up to see what issue they had. Sometimes, the negative reviews are with specific model monitors and not the cable at all.