If you check you'll see that it does have ethernet, so I'm not sure what you're talking about.
Perhaps you're referring to airport/802.11? If your iMac G3 doesn't have 802.11 there are options. There's the official Apple solution, which is only 802.11b and is expensive due to scarcity. There are USB adapters, but one must be careful to get one that has mac drivers (and actually install those drivers). Perhaps the best solution are 802.11 "game adapters" that connect to the ethernet port.
I picked up a game adapter made by Zoom on sale for about $35. It's nothing special, but it gives me 802.11g on OSX and/or OS9 without having to worry about drivers.