ThunderBolt 3 to Firewire 800? I have several FireWire 800 Hard disks full of data/video/images I need for work. Last week I bought a brand new iMac and only today I realized that Apple is not selling any Thunderbolt 3 to FW800 adapter?

I couldn't believe it, and I hope there is a solution otherwise I will switch away from Apple forever.
Buy a drive caddy for $22.00 (USB3).
Take all your old Firewire drives apart pull each disk out, and slip them into your modern caddy. Plug the caddy into USB and pull your data off as needed.

Don't toss the drives. You can use them for backup media etc. through the caddy.
Be sure you do a modern reformat on the drives, and run Disk First Aid.