It's unlikely you'll find a used boom for your Hinterhoeller 28, but a suitable extrusion can be had fairly easily, depending on your location.
Yes, a wooden boom is as reliable as an aluminum
one. In many cases, wood will be lighter and more durable then aluminum
. This is particularly true of smaller diameters and/or well "shaped" spars.
Your boat came, as you expected, with an aluminum extrusion tube for a boom. I didn't look up the spar dimensions, but if you like I could look up the replacement or similar extrusion from Dwyer Masts, though just a guess I'd suggest a DM-375 section would be as big as you could possibly want and it costs about $21 per running foot.