The new pathfinder and XV88 kits come with options for marker drones as well.
XV88 for example has two drone "heads" (the disc art), three antennas and different options for the lower part (2x missile drone, 1x gun drone, 1x (new) marker drone (slightly different to the one from the XV25), 1x shield drone)
For pathfinders it's similar: 3 discs (if I remember right) and lower parts for gun, shield, marker, pulse and grav inhibitor.
In addition I've seen different tutorials to build your own marker drones by using bits from other Tau models.
e.g.
http://advancedtautactica.com/viewtopic.php?f=21&t=20021