Fmuser.net is a Chinese company offering several models of FM transmitters from 500MW to higher power to 10 watts or more. The least powerful model, the CZH series, is 500mw but adjustable to 100mw. This is to much power to be legal in the US. They do offer some 1 watt models that are continuosly power adjustable so you can turn the power down to near "legal" levels.
If you want to do FM in the US there are a few certified transmitters available. The ccrane model 2(ccrane.com) the wholehouse 3 which I have and am using and is just great! This also has an improved feature that lets you use a mic and audio at the same time...a built in mixer(wholehousetransmitter.com) and the decade which is also excellent(search decade transmitters).
Ramsey makes kits and the fm 25 and 30 B models are power adjustable.
Mark