If you don’t have the physical slip any more, you can check the CRA’s My Account service to see all your slips. You can login to the service here: https://www.canada.ca/en/revenue-agency/services/e-services/e-services-individuals/account-individua...

If you are not able to use My Account, then you can open up your 2018 return file and find it there with the Find tool (magnifying glass). You can login to TurboTax Online 2018 here: https://ww2.2018.turbotaxonline.intuit.ca/secure/interview