Where can I buy a repair manual for my car?

https://www.helminc.com/helm/Result.asp?Style=helm&Mfg=GMC&Make=BUI&Model=REGA&Year=2017
Is the OE manual company, however sit down before you look at the price? What I would suggest instead would be to use this site and get one of the subscriptions online for either Alldata - https://www.alldata.com/diy-us/en or
Mitchell - https://www.eautorepair.net/Marketing/Default.asp
It is much cheaper, and you get the same service information as a shop would. Plus, the online systems get updated while a print or copy doesn't.