where can I download

.vxd files are created during driver installation. So, you cannot download just the .vxd. You will need to either re-install the driver, or download an updated driver to install.

Start at www.driverguide.com
There are links there to most card mfg.
And user posted drivers.
If you do not know who made the sound card, look at the card itself.
I suspect it has an ess sound chip on it. Copy down the part number on the large IC. If the logo/name on the chip is not ess, then it is a different mfg. Maybe Crystal or OPTI. Just a different link from driverguide.
You can download "reference" drivers from esstech
Just be sure to get the correct driver for your chipset and OS.

In addition to the manufacturer site and the Driverguide.com site, you can try:
http://www.drivershq.com/
http://www.mrdriver.com/
http://www.driversplanet.com/
You will need to physically look at the chip set as TheChas suggested to make sure that you're getting the correct driver. Look for the latest driver version available, install it and check the .vxd file after installing for that exact version number to make sure it's the one you want. You may have to force Windows to use the new drivers, it may say that the best driver is already installed. Good luck and best wishes!