Eurway no longer sells replacement. See the email the sent me:
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Thank you for your inquiry,
We are no longer selling replacement straps for the Wassily chair. We apologize for any inconvenience.
Best regards,
Eurway Ecommmerce.
Total cost based on Angel77′s answer:
4 x $145 + 2 x $175+ $245 = $1175
That’s not cheap. Actually I could buy 5 chairs at Eurway for that price!!!