Oh apparently bodylastics (the bands I have) have a bar as well
bodylastics Patent Pending Collapsible Resistance Bands Curl Bar with Steel Pipe Construction, 3 Points of Connection, Sharp 45 Degree Angle Bends, Amazing Rubber Grip and Carry Bag. https://www.amazon.com/dp/B07V6Z3171/ref=cm_sw_r_apan_glt_fabc_HW3RFK7SH1T800DH53H0
Not quite what you are looking for, but this has rings already attached. https://www.amazon.com/gp/product/B07V6Z3171/
bodylastics Patent Pending Collapsible Resistance Bands Curl Bar with Steel Pipe Construction, 3 Points of Connection, Sharp 45 Degree Angle Bends, Amazing Rubber Grip and Carry Bag. https://www.amazon.com/dp/B07V6Z3171/ref=cm_sw_r_cp_api_glt_fabc_RPAXDAJW8KKY0CNE9TD7
It’s not a “smart” curl bar, but seems to be popular among Tonal users. https://www.amazon.com/dp/B07V6Z3171/ref=cm_sw_r_cp_api_i_S9NvFb9QTHP2P
Most people probably get them from amazon. Things like this.
https://smile.amazon.com/gp/product/B08G4NDGMW/ (nice)
and this
https://smile.amazon.com/gp/product/B09GL1Z6YJ (not sure I'd get this one again, a little too long for me)
and this
https://smile.amazon.com/Bodylastics-Collapsible-Resistance-Construction-Connection/dp/B07V6Z3171
Some people get ankle straps.