I discovered a few more things about what parts/accessories Costco will (or will not) sell to me.
rasmus is right. I checked. They will sell Bluetooth accessories to anyone. Though, remember these are one generation back.
They do not sell speaker wire receivers. No worries if you bought your hearing aids (HAs) from Costco and they are still under warranty. They will replace them free. If the warranty has expired then they can still replace the receiver by treating it as a repair for $135 per Costco purchased HA.
They do not sell supplies such as Domes and wax traps when you did not purchase your HAs from Costco. No worries if you bought your hearing aids (HAs) from Costco. You can order Domes and wax traps for your Costco purchased HAs.