The Infinite Timereaver is a mount tied to the Timewalking events which takes place every 3-4 weeks. During the event week, players can queue for old dungeons and get their level scaled down to the matching level, and get loot. The Reins of the Infinite Timereaver is a drop from these instances, and can actually drop from any boss in the entire event. This means it can be gotten independently on which timewalking event is active, so it does not matter if it's TBC, Wotlk, Cata or even MoP.
The drop rate is very low, and some people suspect it is 0,05 % per boss, just like the Sha of Anger mount. This means you will need to farm a lot of these instances before you are likely to see one drop for a party member.
The only way to farm this mount is to run the dungeons as much as possible and hope to get lucky. Most players with this mount report that they have bought both the 5,000 Timewarped Badge mounts, so expect to spend a lot of time farming the Reins of the Infinite Timereaver.
In order to farm it as fast as possible you should search for a 5-man party that will chainrun the instances to get rid of queue times.