You can order the Seresto collar online but yes the Seresto collars do require a vet prescription before you can buy them offline or online.
You'll need to have your vet write a prescription for the Seresto collar and then you can upload the prescription when you order the Seresto collar.
The Seresto flea collar is more expensive than other collars and can cost up from $54.00 per collar and even more but many people have given the Seresto collar great reviews.