Just a couple things to consider when looking for an FM transmitter:
To begin with, most of these inexpensive units operate in the micro-watt (uw)pwr range. That said, the antenna system becomes very important if any appreciable range is to be obtained. It's very important that the antenna is tuned to 1/4 wavelength of the FM band, which = 28.5". Also, the antenna needs to be referenced to 0V, which can usually be accomplished by placing the xmitter on a metal ground plane, such as a filing cabinet. Antenna height is also important since FM is pretty much line-of-sight transmission, so obviously higher is better, in most cases.
Unfortunately, even under ideal conditions, the very low amt of output power limits their operating range (usually around 100-300 ft or so). In order to xmit at greater distances, the only practical solution is to use a more powerful xmitter. Besides being more powerful, these xmitters are more costly, and may require FCC licensing. The main issue with using higher power'ed xmitters, is not so much the output power, but rather it's the "Signal Field Strength", as measured at a given distance from the antenna. The FCC has a program called LPFMT (Low Power FM Transmission), which can be referenced on their web-site, for more info.