Can I still get Shadow Links without Streetpass?
From memory yes, you can find Shadow links without streetpass.
There's a couple of them directly inside the game that you can find on specific spots.
Occasionally, a Shadow Link will appear without being met by StreetPass. This occurs once every few hours of gameplay, as long as Link has visited the StreetPass Tree at least once. These Shadow Links come from a supposed Player named Shade and are selected at random from the following list of 20 pre-generated characters, coming in a variety of bounties and item selections.
You can find a list on this wiki here