You lucky fella,
I lived in Mito for two years. I'll probably catch you in Iwama but for now, here are those directions:
I think the one you mean is actually the general martial arts shop which especially caters for kendo, but they also have keikogi
and weapons for aikido and other arts.
From the north exit of Mito station you need to go exactly
straight ahead. This is actually the trickiest part because there are three branches forming an intersection here (which you shouldn't take) and a number of side streets. The road you want is the narrow one between the block where UFJ is and the San no maru hotel which you can see on this map here:
From here, if you're on foot, you need to walk for about thirty minutes. You'll cross the Naka river then you'll see a couple of gas stations but you need the one called "Cosmo" which you should eventually find on your right. The shop is right next to that. They usually have a manikin wearing kendo armour and holding a shinai outside.
If you want any more info, feel free to send me a pm.
Best of luck