Try Microchip, they make a range of RFID chips and supply aplication cicuits.
**broken link removed**
Quote:
Microchip offers a broad portfolio of low-cost and high-performance passive RFID tag ICs. These devices are available with features such as advanced, high-speed anticollision algorithms for reading multiple tags in the same reader field, a unique "cloaking" capability which minimizes the detuning effect of adjacent tags, extremely low power consumption for extended read range. Contact-programmable OTP versions and Contactless-programmable/rewriteable versions are available, with memory density from 128 bits to 1Kbit of user memory