Any car parts store should have it or be able to get it if you give them the make/model/year of your truck. Make sure when they pull it up on their screen it is ACDelco G1852 and when it is handed to you it looks like the picture. There is a vendor called Lectric Limited that people like for that part as well.
Mine was from Amazon.com
https://www.amazon.com/gp/product/B000C9SM22/.
I have modern heads on my engine and the hole is 3/8NPT instead of the stock 1/2NPT size, so I had the sensor turned by a machinist. There are some machinists here on the forum who provide that service for us. Picture by forum member B. W.
Info about turning the sensor
http://67-72chevytrucks.com/vboard/s...=703388&page=2.
If you need that service, Brian Mac says he gets the part locally, turns it and ships it, so you could have shipping one way. A1971Blazer offered the same. So that would mean you don't need to get one first. I already had the sensor, so I shipped it to A1971Blazer. This is depending on what size hole in the head you have. :-)
When I replaced mine, I loosened the radiator cap, removed the old sensor and held my finger over the hole while I got the new sensor in my hand. I only lost a cup or 2 of coolant.
You only need a single wrap of teflon tape on the sensor, don't go overboard with it. Some people say you don't need any.