AJATA Einradversand
AJATA is a unicycling vendor only recently established. Together with Municycle.com they have the majority of the market share in Germany. Remarkably AJATA and Municycle.com are the only German unicycle vendors I know that are run by active unicyclists. That of course has implications on their competence and quality of consulting.
DM
DM was a one man company form UK that built high quality unicycles and giraffe unicycles. But the quality raised impacts for the costs. DM is my personal favorite for large (>> 2 meters) giraffe unicycles. My own 3m giraffe unicycle is a DM one. Unfortunately DM is no longer available in the market.
Miyata
Miyata is a Japanese brand. For long time, it was said (I think for good reasons), they build the world's finest unicycles. But there's one problem: Miyata produces for the Japanese market and doesn't consider that average Europeans or north Americans are some taller than most Japanese people are. Nevertheless, Miyata unicycles provide a great starting point for modifying them to fit demands for high quality. I have two unicycles that once have been Miyata unicycles, even if it's now hard to see that. But many standards (tube diameters, bearing sizes, ...) have changed during the last decade, while Miyata still sticks to the standards from 10 years ago. As a result many current spare parts don't fit Miyata unicycles.
Municycle.com
Municycle.com is the German subsidiary of Unicycle.com run by Roland and Petra Wende. Unicycles and parts they cannot get for you, are probably not available at all. It is very worthwhile to rely on their competent and friendly consultation. Last but not least I want to mention the delicious gelly beans, available in the shop.
Osell's Bicycle/Unicycle Shop
Osell's used to be a bike shop in Minnesota. Some years ago, their frame builder Paul Wyganowski was asked by some Minnesota unicyclists to build unicycle frames. They wanted custom frames for people, whose needs exceed a standard unicycle. Meanwhile unicycles and especially excellent unicycle frames belong to the stock. The frames are still custom made and won't let any wishes unfulfilled. And if there are any special needs, just let Paul know. He will manage it.
I want to post a nice story here: when I ordered a frame at Osell's, I discussed it pretty long with Paul on the phone. Finally we agreed how to build it. As often in life, I forgot something. In this case it was the threads for the wheel brackets. In the USA, inch threads are standard. But I wanted them metric as it's not easy to get inch screws here in Germany. As mentioned before, I forgot to tell Paul. You can guess how surprised I was, when Paul made them metric anyway.
Pichler
Kurt Pichler from Ludwigswinkel (Germany) builds anything ranging from normal unicycles, over giraffe unicycles to recumbent bikes. Special requests are not a problem. I recommend Pichler's giraffe unicycles up to a height of about 2 meters. I'm riding Pichler's "Travel" giraffe unicycle.
Semcycle
Semcycle is a Dutch manufacturer, lead by the Abrahams family (Alberto, Sem, Theresa and Juri). There's a wide range of opinions concerning Semcycle unicycles. They are loved or hated. My opinion of Semcycle's product philosophy is somewhat sceptical.
Siegmono
Siegmono is a manufacturer and trader near Kiel (Germany). Georg Siegmon was one of the first vendors in Germany, who offered a rich variety of spare parts. For long time it was the only shop in Germany selling Miyata unicycles. Some of my unicycles resp. parts of them have been provided by Siegmono.
Unicycle.com
Historically Unicycle.com is a US vendor who offers nearby any unicycle, produced anywhere in the world, including spare parts. Emerging from that a worldwide net of unicycling vendors has grown. Also a German shop is available. A special gimmick is that for all products there are very nice pictures available on the web page. So you can have a look at the stuff.