how can i find?

You counldn't spare even one minute with Google? This is what I found on the first page of results toa search for "1600 8v 2003 Vauxhall astra thermostat"

http://www.diynot.com/forums/automotive-repairs/vauxhall-astra-16-extremely-high-temperature-help-needed.321165/

https://www.youtube.com/watch?v=EiJUNzK4KlI

Sheesh
June 2015