Actually, I am indirectly involved with this industry so I know a few places. Sanwa is HUGE and they are located in the Kojimachi area. Even though its a big company, their website blows donkey balls.
For some reason, bunch of these rental companies are concentrated in the Shinjuku-Yotsuya-Ichigaya area. Another one I know is Circle (http://www.circle-j.co.jp) and they are very good knowledgeble people. I could also recommend TECs <- I don't know their url so please google.
If you need specific contacts, then you can PM me and I'll provide you with the information.