Those names of the last two stones were not those given to them by Joe Dever... it also seems rather unreasonable that the Elder Magi would place a Lorestone in Xaagon.
Anyway:
Varetta - in the Tekara Cathedral as you say.
Herdos - kept by Zahda in some sort of force field together with a Doomstone above his throne.
Ohrido - where the Elder Magi left it in their temple. A bit unclear exactly where, as it materializes in LW's hands.
Tahou - according to legend, thrown into the Tahou Cauldron by the Anarians. There guarded by the Crocaryx of Zaaryx. Also a bit uncertain exactly where it is kept as it materializes in a similar way t the one in Ohrido...
Luomi - captured by Gnaag during the Hammerland invasion of Eru and kept in an energy "fountain" above the shadow gate of Torgar.
Gamir (Darke) - captured by Gnaag, also in Torgar.
Vinaldo (Sphydar) - captured by Gnaag, also in Torgar.
Of the last three stones, LW only manages to grab one in Torgar; a second he snatches from a scarlet clad warrior who picked it up where it landed in the Daziarn, in the Grand Sepulchre in burial grounds of Tolakos. The last he takes from Vonotar after killing him in the Temple of the Sandai in the city of Haagadar.