Hi @jillstjohn
Ifixit does not do repairs.
It provides a forum so that users can get/give information on how to fix their devices themselves.
Here's the ifixit [[Topic:DYMO LabelWriter 450]] guide that may help you to disassemble the label maker so that you may be able to unjam it etc.
Searching online for parts for the label maker only gives results for spools and printheads for the device so it may be difficult to find the "broken plastic piece" replacement part. You may have to contact Dymo in your location and ask if spare parts are even available for your model.