The original version of the WRT54g v1.1 used a 5VDC 1.5A adapter. The current versions all use 12VDC 1000mA adapters. The outer ring is negative. The center pin is positive.

The WRT54G will tolerate a very wide range of DC input voltages. I've tested mine and it runs on anything between about 4VDC and 18VDC. I would not go much above 18VDC. The switching regulator can take it but the input filter capacitor might not.

I'm currently running the WRT54Gv3 that's sitting on my server pile on a 9VDC 1A power supply that appears to have come from the junk heap. In other words, almost any voltage will work as long as the adapter can deliver at least 1A.

The connector is 5.5mm O.D. and 2.5mm I.D. Radio Shock type "N" plug.

The original part is a Linksys AM-1201000D41

Go to eBay search and try "12v 1a" or "12v 1000ma". You have 75 to chose from.