Actually Mission Models is sold out of them as well as AAA Hobbies, Houshinya's other US distributor. I've contacted Hiro myself and he said that the company that was making the sets quit due to low demand. He is currently looking for a new company.
There is another guy based in Germany named Schatton. He also produces turned metal parts. Currently, he has the 2.0 barrel and antenna in one set and torpedos in another. I contacted him about possibly producing scopes as well as both barrels for the u boat, but he didn't respond. Maybe if enough people suggest it to him he'll consider it.
I was lucky to get a set from Houshinya for my first U-boat and they really look great. I 'm currently building a second one and I don't know what to do about the scopes and barrels. After having Houshinya's I won't use the kit's. It will have to remain unfinished until someone produces them.
Here's a link to Schatton's site if you're interested:
http://modellbau-schatton.privat.t-online.de/