It's these things.
There’s a couple options:
The game Ring Fit Adventure comes with a leg strap, so if you have access to that you’re sorted
You can get an off brand leg strap from Amazon or Ali-Express. These were made for Ring Fit Adventure, but it will work for NSS. This is one from Amazon. There’s cheaper options as well
On the same day NSS comes out, Nintendo is releasing a standalone leg strap accessory for $20NZD/$13USD. I’m not sure what country you’re from but this is the pre-order page from a New Zealand tech shop