3rd party Squarespace templates
If you want to buy 3rd party Squarespace templates, Squarestudio is the only shop I know that sells actual templates that install on their own. They are cheaper than most design kits and you can customize them like you would any other Squarespace template.
They also have a membership, which means you can get access to all of their templates for about the price of one.
Here's how the install process works
Check out their template collection here and use the code SQSP10 to save 10%