A company called onlinemetals.com has 20mm rail with a 5mm slot (compatible with rail-buttons.com mini buttons) for a much less eye-watering price than 8020.net. I was able to get a 6 foot length for $39.84 delivered. It arrived today, and I was able to verify the buttons fit.
https://www.onlinemetals.com/en/buy...m-slot-:-t-slot-aluminum-6063/pid/mp-00001377
https://www.onlinemetals.com/en/buy...m-slot-:-t-slot-aluminum-6063/pid/mp-00001377