Since the vet mentioned it previously and they have Bernards weight on file, could you ring and ask them to decant some panacur for you to pick up?
It is a wormer used for a lot of animals and available in different forms (liquid, paste, granules) & strengths. I expect the feed store would have larger bottles intended for farm animals. That’s actually what my vet uses and they portion out what’s needed to treat a rabbit.
I know in the UK, pet rabbit owners tend to regularly do panacur treatments as the buns are either houses outdoors or are indoor/outdoor. The product usually indicates 9 days panacur every 6 months. However, treatment for known EC is 28 days. I do know or some rescues there that routinely treat incoming rabbits with the 28 day course.
I will mention that panacur (fenbendazole) can cause bone marrow suppression so the 28 (or 30) days is carefully prescribed as a balance to treat active EC infection but not cause more harm. Some vets will do a blood check 2 weeks into treatment. Then another at 4 weeks. Or they will check after the full course if the rabbit needs to have more panacur. I am not sure this is common practice though, to be honest.