Low Ab

You could buy a Hohner Thunderbird (~$110-$140) in Low G and then tune it up a semitone (a pain) or pay someone to do it (more $).

You could get Seydel to make one as a "configured" harp - Low Ab is one of their standard special configurations. Check out their custom page at http://www.seydel1847.de/epages/Seydel1847.sf/secc91c96fba0/?ObjectPath=/Shops/Seydel/Categories/Configurator/Config_Harmonikas

Looks like a brass-reed Blues Session would be $60, a Session Steel would be $80, or an 1847 Classic or Silver would be $110.

I don't kow what they do about reeds hitting coverplates. The best out-of-the-box solution is obviously the Thunderbird. You could discuss this with US Seydel rep Rupert Oysler; his contact info is on the Seydel site.

I sometimes try to figure out where the reed tip will contact the cover, and then hammer out a small dome from the inside of the cover to add some clearance, using a steel punch from the hardware store.

Kinya Pollard here at bluesharmonica.com may have additional suggestions.