Need PCI-X card for my Workstation that works with Windows 10

Hey guys,

I am a teacher/IT guy and I host a gaming club at our school. My workstation at the school I work at is used to host our site licensing server(Steam Caching) as well as run VR. Quite a lot of load for one box but it handles it well. The unfortunate part is I have absolutely no network bandwidth to work with and we want to stream gaming club to twitch so parents and friends can tune in and watch the VR gameplay.

To alleviate this I'd either A, completely swap platforms costing several hundred dollars, or...
B: Get an affordable PCI-X cards networking cards.I have NO free PCIe slots, but I have two free PCI-X slots on my motherboard.

Asking you guys if you know of any PCI-X cards that work on Windows 10. Ideally I'd like a multi-port NIC. The more ports, the more bandwidth. And before you tell me it's overkill, I have to go this route because the networking coming to the room is only 10/100(School ain't buying me a network switch haha), so the more ports, the better. If it were gigabit, this would be far less of an issue.

Thank's in advance.