Depends on your exact needs but there are many sources for "RTD extension wire." It comes in 2,3,4 wires to a cable up to 900 degrees F. So you need to be a bit more specific about your needs.
We used to use 3 wire 18 AWG shielded. Now we install 4-20 transmitters on RTD's. But any quality wire will work. It's a resistance so in long runs
you need large enough wire. And good solid connections. The connections would more likely be a weak point in the system than the wire.