Hello hayrider - I purchased one of the occupancy sensor bypass modules you're referring to from super user CTScott. I replaced my factory seats for a set of sparco's and wanted an elegant solution for the passenger airbag/seatbelt situation.
The bypass module comes with a switch that you can install on a dead panel. The switch allows you to turn the passenger airbag and seatbelt on when someone is riding in the passenger seat and off when empty. It's a fantastic little device that works with many different Toyotas due to Toyota using the same SRS module in a variety of models.
As a first step, I'd reach out to CTScott with a private message here on the forum, as you might be able to score one from him shipped for less than the vendors he provides them to online. He's super busy, as you would imagine for someone so talented, so if that doesn't get you anywhere, you can purchase the module at retail price
here.
Hope it helps and make sure to follow-up with pics of those new seats once you get them in.
Edit: I looked closer at the website I linked you to and unfortunately, it says they are not accepting new orders for that item and aren't sure when it will be back in stock, if at all. I'll keep my fingers crossed for you being able to get in touch with CTScott about purchasing one directly from him.