I bought a USB 2.0/firewire PCI card for my sawtooth several years ago.
At that time, OS X wasn't mentioned as a supported OS. but the card works fine.
Now I find OS X is listed as a supported OS on essentially all the USB 2.0 PCI cards I have seen. I would think you can get that type of card anywhere - think what else you might need on ports and get a combo card if available.
I'm using up my PCI slots a lot faster than I thought - basically because I like everything integrated in the Mac.
The one thing you will want to keep in mind is that some USB cards comply with OSX's sleep commands while others do not. The card I have (MacAlly) allows my machine to sleep but only if nothing that uses power is plugged into it. An extension cable or a hookup to a hard drive or scanner that is off won't matter, but a USB key (even unmounted) will pretty much make the machine unuseable if put to sleep.
I have no idea if you can even get one that allows for proper sleep with peripherals plugged in. Then again, if you don't put your machine to sleep it won't be an issue.