260 - 500 C°
For this temperature range, you can use either a resistance temperature detector (RTD) like a Pt100 and Pt1000 or a thermocouple. RTD are generally more precise than thermocouples, but thermocouples can be built in smaller diameters.
With regards to the insulation material, the best choice is a mineral insulated cable probe. Fiberglass insulation is also suited for this temperature range, but please keep in mind that fiberglass is hygroscopic.