This guide will let you know how to find your purchased tank in GTA 5 Online Not In Hanger.
The first thing you will need to do is hold up on your d-pad and then go to your phone. You have to open your contacts and call Pegasus.
When you call him, you will find some options on the left top side of the screen, you have to select the Tank option, and they will deliver the tank to you wherever you want.