If your from Europe, you can order Ivermectine from this shop:
and search for eqimectin 800.
During the payment process, select pick up by postal service and let the prescription be done by the shop.
It works, I have 6 tubes now, maybe you could also order international, but there's customs coming into play.
Edit: fixed the link.
How to order Ivermectin in the USA:
Valley Vet:
https://www.valleyvet.com/ct_detail.html?pgguid=3A1926D3-77D8-4AF0-BB6A-AC29492A0DD5
https://www.valleyvet.com/ct_detail.html?pgguid=4D9A49D8-394F-46B3-883C-733D88C2C257
https://www.valleyvet.com/ct_detail.html?pgguid=2E87BED9-7B6A-11D5-A192-00B0D0204AE5
Jeffers Pet:
https://www.jefferspet.com/products/jeffers-ivermectin?via=5338848b9fa2600f00000196%2F533884929fa2600f00000221%2F533884949fa2600f00000257
https://www.jefferspet.com/products/bimectin-paste?via=5338848b9fa2600f00000196%2F533884929fa2600f00000221%2F533884949fa2600f00000257
https://www.jefferspet.com/products/zimecterin?via=5338848b9fa2600f00000196%2F533884929fa2600f00000221%2F533884949fa2600f00000257
You can search for same products on other vet supply shops.
As you can see, it is very cheap. Average $5 for 5 doses at 250 lbs.
*Make sure products contain ONLY 1.87% Ivermectin and no other drug. There are many dewormers with multiple drug ingredients. You want ONLY Ivermectin.