Are you sure that you have a bad video card and not some other problem? What are the symptoms?
Check the dip switch settings on the motherboard, where it is labeled SW1. I think the display card, the switches and the video monitor all have to match. The settings should be printed on top of the disk drive, after you take off the cover of the computer.
This model could be configured for VGA and monochrome monitors, so you may simply have to adjust the motherboard display setting. What kind of display are you using? Is it an original? Does it work on any other old computer from the same era?
Any early VGA (1989-1996 era) video card should work. If you can't find a Compaq-branded card from Ebay, go to your local Thrift Shop and see if you can find any 386/486 computer. You should be able to do a transplant.
Want a secret source for stuff like this? Your local computer repair shop will usually have old VGA cards lying around and would be happy to sell you one for $15. The added benefit is that you can bring your system in and test it right there.