Not sure the OP is after a Japanese dictionary itself, Andrew... more just something that can help him grasp the terminology used in his dojo. And, for the record, conversational Japanese isn't really the same thing... there are certain terms that you'll encounter in a martial arts dojo that you won't in regular Japanese. And, to that end, Paul is right... he won't find what he's looking for. Each different art, and in some cases, each different dojo, will have it's own heigo (martial language)... if you walk into my dojo, and say "Osu!!", you'll get a funny look, maybe a slight smile, and be told not to say that silly little phrase here... head into an Aikidojo, or a Karate-dojo, and it might be heard some thirty or forty times in an hour from each student... and each might have a different interpretation of what it means, and how it should be used.
You might attend one dojo (Aikido) where the two training partners are called Uke and Nage... then, in a Judojo, they're known as Uke and Tori... in Tenjin Shin'yo Ryu, you'll find the terms Ukemi and Torimi... in some classical arts, you'll hear references to Aite and Ware... or Teki... or Uchidachi and Shidachi... Kiri-komi and Uke-komi... and this is just an inexhaustive list of terms for the partners in practice, without getting into everything else...
That said, if I was to recommend a book, it'd be David Hall's Encyclopedia of Japanese Martial Arts... it's not really going to give much in the way of dojo terminology (although there is some there), but is a great reference for many arts and concepts of Japanese martial systems. Other than that, it'd be best to search for a glossary specific to the art the OP is after... which will be different to the lists of other dojo and arts...