I checked eBay, they have yet to put in a separate category for these converter boxes The closest that can come to it is: Electronics > A/V Accessories & Cables > Television Parts & Accessories > HDTV Receivers & Tuners

There should be a "Digital-to-Analog Converter Box" category with this map:

Make:
Model:
Power Source:
Digital Format (ATSC, DVB/T, ISDB-T, DMB/T, Multiregion):
Analog Pass-Through (if RF output included):
Video Outputs:
Video Resolution:
Audio Outputs:
Audio Frequency Range(Hz-kHz):
User Interface (types of graphics mode used):
Program Guide:
Manual:
How Upgraded:

Unfortunately, it's very hard to test the actual frequency range put out by these boxes - I fear that, in addition to picture quality seen in several boxes I've tested, the audio range put out by the boxes could be chopped.

There is a DTV test pattern generator that includes an audio frequency generator out there, ATSC RF Generator, 8VSB QAM Signal Generator - Leader Instruments , but it would be very very expensive - just see how much an NTSC/Pal monoscope generator goes for! Leader Instruments Corporation Leader Monoscope NTSC or PAL Test Signal Generator Color Bar & Pattern Generator at Markertek.com