The only place to get invisalign aligners or retainers is through a dentist who has been trained and certified to do invisalign. Invisalign make a product called Vivera where they send you a new retainer quarterly for the year. You must see a dentist though
The only way to get Invisalign aligners or retainers is via a dentist. The retainers are essentially what dentists call Essix retainers, and most dental offices can provide them as well, even if they are not Invisalign certified.
More or less, there is no getting around using a dentist.
Invisalign offers a retainer system that makes you several sets that are intended for use over 2 years time. These must be purchased form an invisalign provider. Clear retainers are called essex retainers and came out long before invisalign. They can be made be any dentist or lab. See Johnsdental.com