Actually, the electromotive force (EMF) is generated across and by the junction. Therefore the voltage is across the junction, not the wire. There is only a voltage across the wire if the wire has non-zero resistance and is part of a complete circuit, otherwise the end of the wire is at the same voltage (potential) as the junction end. Even then the "voltage across the wire" is a dropping voltage, not a source voltage (EMF) like it is across the junction.Actually though, the voltages are not generated across junctions, the voltages are generated across the lengths of wire.
Contact between any two dissimilar materials creates an EMF. Copper and iron, ruby and cat fur, ... all generate an EMF when in contact. If the materials are conductors then they can be used as a voltage source, albeit typically a very weak one. In the case of a thermocouple, the junction is a temperature sensitive EMF source, hence the use of thermocouples as temperature sensors. Any two (IIRC) dissimilar metals can be used to construct a thermocouple. For contact between non-conductive materials you get a static charge that is generally unusable in an external circuit since the materials of the junction are non-conductive. But you still get an EMF source at the junction (contact surface) between the two materials. For non-conductors it is often possible to get extremely high voltages, thousands of volts is not uncommon, across the junction.
For thermocouples, a sensitive (and high impedance) voltmeter is all that is needed for the basic thermocouple circuit. No external power source is required to operate the temperature sensing device. Indeed, that is all that is in many thermocouple based thermometers; the junction and the meter.