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Japanese Floor Mattress Futon Mattress, Thicken Tatami Mat Sleeping Pad Foldable Roll Up Mattress Boys Girls Dormitory Mattress Pad Kids Floor Lounger Bed Couches and Sofas, Grey, Twin Size [link]
Where in the US do you live? If you live near somewhere with a large Korean diaspora community go look for a store selling 이불 (yi-bul) and ask if they have yo mats. They will probably be expensive.
Another option is getting a Japanese mat from amazon like this or this.