Buy it at pet stores in the bird department. It's usually cheaper in the bird department than in the reptile department. Online might even be cheaper yet. Even cheaper if you buy in bulk. Amazon has 5lbs for around $30. It might be about 50 cents to a dollar a piece if you buy it singly.
My first week with my tort she woke me up in the middle of the night crunching on her cuttlebone freaked me out the first time and I jumped out of bed to make sure she was ok! We are on a better sleeping pattern now she seems to love them!
I leave cuttlebones in all my enclosures inside and out for my leopards, stars, and greek. They always eat them eventually. It's good for their beak maintenance as well as a great source of calcium. You can get packs of 6 in the bird department at Petco, though i'm not sure if you have access to one of those stores? And I also offer egg shells without any issues in my animals for the last 5 years I've been keeping them. I always rinse them really well first but most of the tortoises will just straight up eat the shells.