You might try calling Gander Mountain, I don’t see the ducers listed on their website but they do have the transducer arms listed as in store so they might have the ducers in stock. I’d also try calling Joe’s Sporting Goods in St. Paul, they carry them. Looks like Cabelas may sell the tri beam ducer in stores but they’re currently out of stock.
Marcum is located in New Hope, you might be able to walk in and buy one. Not sure though.