A retailer in Germany is selling 173 mm Shimano skewers on eBay for the equivalent of a few dollars (intended for internal gear hubs, it looks like.) This is OK for a tandem, although 178 would be better. But courier shipping is like $100 ! Since COVID, many European retailers have shifted to couriers for their North American deliveries owing to mysterious "losses" in the mail and demands for refunds, especially early when flights were full of travelers trying to get home and then stopped altogether. But at that price, it seems they don't want to be bothered with North America at all, since it's about double the charge I've paid for other couriered orders from other places. Actually ordinary tracked Royal Mail and French Post are fast and cheap but it's still hit or miss as to whether a seller will use the mail or not across the ocean.
The other thing you could try is contacting the seller (through eBay) to ask if the shipping charge is correct. Sometimes the automated shipping calculator gets it way off and the seller will say, "Goodness, no, not nearly that much."
I bought two 173 mm XT skewers from a guy in Taiwan a few months ago, gave one to a friend. Haven't seen any on eBay since, other than these from Germany, which aren't XT, by the price. I've had a search for the better 178 mm skewers on for the past year, not a single hit.
Sometimes a bike shop will have some long skewers with internal cams sitting around in a drawer. Can't hurt to ask.