OR's web site says they're made of Delrin (acetal thermoplastic) The common aluminum or fiberglass tent poles probably won't flex tight enough, but I'm not positive about that.
Years ago I made a bivy with a similar hoop arrangement. I used "music wire," sometimes called "piano wire," and joined the sections with metal tubing. I don't remember what diameter, but it was less than 1/8".
Many hardware stores and hobby shops sell it in 3ft. lengths.
You can probably order the original poles from Outdoor Research.
You can buy Delrin rod here:https://www.aircraftspruce.com/catalog/mepages/delrinrod.php
You'd have to fashion some ferrules from metal tubing and epoxy them in place.