The plastic clips come with the unit which is manufactured by Dometic Corp ( dometic.com ).They have both a dealer look up page and a contact page.
I would suggest leaving your message and contact information on the contact page and see if they will just send you some.
Plan “B” might be to find a dealer in your local area to see if they have some in hand.
These clips are attached to the vent when it comes to Intrepid so we have never had any loose ones. If you strike out on either of the other plans let me know and I will see if I can round some up for you.
In the mean time I would suggest taping the vent closed. The idea is to prevent damage but not shut off the air flow. If seeing tape is too much you could try wedging it shut with some folded paper or something of that nature.