Where to buy holga iphone filter case?

i was looking for the holga iphone filter case and i found it on amazon but for some reason some of the entries cost significantly less, like $5-10, even though i'm pretty sure the average retail price is $25-30. i obviously would want to spend the lesser amount but i don't know if it's less because it's faulty or from a bad source, so how can i make sure the quality is good? it looks the same as all the other entries and i obviously wouldn't want to spend $30 on something that i found and was $10. any help at all would be great, thanks!!