Although they have still not releasesd pricing, there is at least 1 company who has achieved a TSO'd GPS receiver which would plug into the standard 9-pin DSUB NMEA connection to the garmin or avidyne mode s transponders:
There are non TSO varaints of the same thing in the $200 range from Dynon for the skyview system (small combined waas-enabled gps receiver and antenna that will wire in to the transponder through std nmea ports). It is still unclear (at least to me) if that would be acceptable for an EAB - it seems clear that you would not be able to shoot precision gps approaches without TSO - but general ADSB?
I hope we will not *have* to buy a full GNS430 or 530 TSO GPS to comply w/ the ADS-B out mandate. But I guess we will see in the coming years how much this truly will cost us.