Horray! My shield drone conversion kits showed up. I ordered 2 of them (makes 8 Gun drones into shield drones). I really feel like they could have included the plastic disks (the top part) for the $9 they cost, but meh. The glue is drying atm, so pictures later (probably painted. Drones are easy).
Here's a link for them: Games Workshop: Shield Drones
They're buries in the menus. They don't show up in Tau bitz or in the Warhammer 40,000 -> Bitz -> Conversion Packs. They're in Warhammer 40,000 -> Bitz -> Misc -> All the way at the bottom.
The only other places to get them are with the XV15 Shas'ui (comes with 1), or Shadowsun (comes with 2).