I got a new iMac 24. I already have a seagate external hard drive with a firewire 800 connector. Where can I get an adapter?

I haven't personally used one of these on my iMac but it looks like what you need: Apple Thunderbolt to FireWire Adapter - Apple

Those drives normally had FireWire and USB connections.

Turn it around and look on the back to see if it has a port that is marked with a USB symbol. If it does, then it should have also came with a USB cable.

If you use the USB cable, then all you need is an inexpensive USB-C to USB-A adapter instead of two expensive adapters to go from USB-C to Thunderbolt 2 and then from Thunderbolt 2 to FireWire.