UPDATE
5/9/2018
Bendix/King, L3 and ACSS released the NXT-700/MST-70B as form fit replacement for the MST-67A. It is DO-260B compliant to meet the ADS-B Out mandate.
There are other options besides the MST-70B to gain ADS-B compliance. There are various STCs that allow the removal of the Bendix/King transponder system and installation of Rockwell Collins or Garmin transponders. Upgrade prices vary depending on actual installed equipment.
Regarding ADS-B for the Garmin GTX-330 transponders, the best solution is to simply have the units converted by Garmin to include the ES (Extended Squitter) upgrade. This upgrade is about $2000 approximately. Additionally, a WAAS/GPS source will need to be interfaced. We can process the upgrade for you or any other Garmin dealer can as well.