First off, best place to look for these are the CD's/DVD's that come with most PC's. If your computer came with disks, the drivers for all your computers hardware should be located on one of those.
Depends on the make and model of the computer, but most manufacturers post all their drivers on their support site. I would first suggest you go the manufacturers support site, select your model and operating system and look for the network section to find the appropriate drivers.
If that doesn't work, you can go into Device Manager and locate the network device (most likely the one with the yellow triangle with an exclamation point) and right-click and select "Update Driver" and search locally if you have it on your computer or via the internet if you have another way of connecting to the internet other than the network adapter you are looking for the driver.
And finally, like rgd1101 said, I would just Google the device make/model and you should be able to find drivers that way. If you can't find the network adapters info, you can also use Device Manager>>Properties to find out that information.