Do orthodontists keep the mold of the first retainers they ever gave you? I lost my retainer today.

You'll have to call the office to see what they have on file.  Many offices keep models of your teeth the day your braces were removed so that replacement retainers can be made, but some offices lack storage space and are unable to keep models for all of their patients. You may need to stop in to have another impression or scan done so a new retainer can be made. I would do this ASAP to minimize how much the teeth may move before you can get a new retainer!