Mid range RFID sensor

Hello,

After finding some RFID sensors in sparkfun, dealextreme... I have nothing clear. I see a lot of sensors that has a range of about 2-5cm. ¿Exist any sensor with pasive tags and a distance of about 15-20cm with a reasonable price? As far I know the size of the tag is important, but I need a low-sized tag (for a cat collar).

Thanks a lot!

Have you tried Adafruit.com? They show a large collection of RFID stuff.