It can be tricky to find a good used one. Many get broken during heater core replacement because some people don't remove all the screws and just start yanking on it. Also, easy to misalign the hidden part of it on the ridge during re-install, warping or cracking it. Have seen more than a few cracked across the flat area from being kicked.
Still available new (for now) about $45 from a lot of online sources, there's a few on eBay, couple bucks more on Amazon, etc. and lots of online GM dealers just watch out for shipping costs.
Not saying it's impossible to find a good used one...but depending on parts truck supply around you, you could waste more than $45 worth of time just looking through all of them and driving from place to place.
Richard