(Resistance Temperature Device): A sensor that measures temperatures with a bridge measurement (a three- or four-wire half-bridge or a four-wire full-bridge). The sensor uses materials that have a variable resistance depending on the temperature of the surrounding area and on the length of the material.
You might find a surplus industrial electronics place has them - locally anyway. While there are lots of them around it would be tough to identify what you had. There are two places here in Rochester that I'd try - not because they carry them specifically, but because some might have come along when they purchased a pallet of surplus stuff. You might check with a local instrumentation sales representative - one that sells process and temperature control instruments. They may have some discards that they'd part with at little or no cost - can't hurt to ask.