Now the problem is I want to collect data in a .txt file immediately after arduino board get the data. Or if it is possible, I want all the data to be collect in spreadsheet.
Saving the data as a file is going to require some kind of file storage device and software. Typically, this is an SD card and appropriate library.
Getting the data into a spreadsheet can involve removing the SD card from the Arduino and inserting it into a PC, and opening the appropriate file using a spreadsheet application. Or, it can involve streaming the data real-time into a spreadsheet application. For Windows OS, this is possible. GoBetwino is one way. There are plugins for Excel that allow it to be fed data via the serial port, in a more direct fashion.
Can anyone suggest any website for buying an insulated box to keep the suitable environment for arduino borad to be put in the high temperature area please?
No, and I wouldn't recommend it. The Arduino should go outside of the hot area, with just the thermocouple wires going inside.