"Where could I get some 300-900 MHz room transmitters with output of 200 mw or more? Please send names of companies that would have these devices. Thank You."

Easy question... but... If you find people reluctant to answer it for you, here is the reason:

The (US) courts have ruled in several recent cases that the person who only "advises" someone about illegal eavesdropping is just as guilty as the person who actually goes out, buys the equipment, installs it, and listens in. (I don't know that this is your intention, but... I don't know that it isn't either.)

This has lead to some very strange twists of fate...
Like the private investigator who advised his client that recording the wife's phone calls was illegal (federal offense) and should not be done. The client said, "No problem, I'll take all the risk and do it myself. Where can I buy a recorder?" The investigator tells his client to go to a radio shack type store, and does nothing more to help.

Of course, the wife finds the recorder and makes a federal case out of it.

The court determines that the wife cannot testify against the husband due to Interspousal Immunity (the common law prohibition against spouses suing each other). The husband walks.

The investigator - says the court - is an accomplice because he aided in the commission of the crime. He is prosecuted, and is found guilty! He is fined and is sentenced to serve time. He also loses his license... his sole means of support.

My guess is that this former investigator wouldn't tell you where to buy a battery now, even if you showed him your pacemaker and were blue.

So it goes. Thank you for raising the issue.

Kevin