I have collected all but two items for the (mage) Tempest Regalia set.
All of the items have been collected in Black Temple.
According to wowhead.com 2 of the items are collected from Hyjal Summit, and 3 items are collected from Sunwell Plateau, but I have never been there. So how come I have only 2 items left to collect??
How can I track the items in-game?
I think the main pieces of those sets (Assuming you mean the tier 6) are from BT and MH. The extra pieces WoWhead lists as part of the transmog were not officially part of the tier set and were added later with the Sunwell patch.
Black Temple
Chest - Illidan Stormrage
Feet - High Warlord Naj’entus
Shoulder - Mother Shahraz
Legs - Gathios the Shatterer, Lady Malande, Veras Darkshadow
Wrist - Shade of Akama
Mount Hyjal
Head - Archimonde
Hands - Azgalor
The two items missing would be the two from MH, part of the Tier 6. The other 3 mentioned on WoWhead would be the extras from Sunwell which were not part of the original Tier 6.
The 3 items from the Sunwell Plateau are indeed part of the Tempest Regalia item set however they aren’t part of the Tempest Regalia transmog set. The reason is they don’t realy match the art style of the set from Black Temple. Your 2 missing items are only the ones from Hyjal Summit. Items from Sunwell are actually a part of Tempest Sunwell Garb transmog set.