To my knowledge, I am not sure whether PCI-E slots are backwards compatible with PCI devices. This being the case, I do not believe there is a manufactured network card for PCI-E simply because it would be a waste. PCI-E is far faster than PCI and the bandwidth on the slot would far exceed what would be needed from a PCI network card. However, PCI-E to PCI converters are on the market and this could be a feasable option if you considered buying a PCI card with only a PCI-E slot available.
Hope this helps.