How can I find or build a passive RFID tag with a range of 6ft, as well as an associated reader?
You're unlikely to get a good distance reading from passive RFID tags required for trilateration. Passive tags get their transmit energy from the reader. To get a distance in wireless systems you typically look at how much power you receive from a known transmitter, in this case you have to guess at how much power the tag got and then reflected back and account for all those losses. Reading passive tags at six feet is also very difficult.

The first paper I published in college was describing a system I designed using passive RFID tags for aiding localization. I used them to update a kalman filtered dead reckoning system, resetting the system's location to within the four inch read range of passive tags. It was rather novel at the time, got me a full ride for grad school at least.

That being said, you'd probably be better off using active tags if they don't need to last years.

Look at the existing literature for such systems, there are a lot of publications on the topic. They will often share details of the implementation that commercial systems hide. Once you have started your design, come back and ask more specific questions regarding implementation.