Your problem I think is thatbthey will have Apple Direct Connect which was Apple's exclusive connection, with the one cord having what looks like a DVI connection, that carried power, picture, sound and USB to the displays. ADC friendly graphic cards have not been available since that period, the G4 PowerMacs series.
It was possible to purchase a mains power adapter, DVI/ADC, whch was a huge, beast of a thing. Last time I had one it exploded in about 2012.
This what what they were 1. Mains to adapter unit. 2. Adapter. 3. DVI connection cable to ADC. 4. USB cable from adapter unit. Lots of folks thought it was a simple matter of a to ended cable, but of course you had to get mains power to the unit.
They are still about as you can see on eBay:-
Genuine Apple DVI to ADC Display Adapter Model A1006 w Power Cable | eBay
And this is the ADC/DVI without power and will not work:-
Apple Monitor Adapter ADC to DVI 30 Male to DVI24 Pin Female Power Mac Connector | eBay