1) - Go to the Device Manufacturer's Site.
2) - Search for it Online.
3) - Search by Vendor / Device code as shown in device manager.
If you go in device manager, (for whichever device you need driver for)
Right click properties for the device, go in details for the device driver.
You will see a code like per example VEN_8086&DEV_27D8&SUBSYS...
The VEN code means vendor and the DEV code means device: in this case the vendor is 8086 and the device is 27D8.
Once you have the two codes, go to www.pcidatabase.com.
Enter the two codes and you should get the name of your hardware. From there you can search for a driver download for your hardware.