This seller has been hawking this card on Amazon and eBay for a while:
I know that Sonnet had an Aria Extreme PCI card (N80211-PCI), which was 802.11n compatible. Not sure if the G3 would be the best match for it as it might have required Leopard although you could hack Tiger to work with it on a G5.
This one is pricier but if you recognise the card on your browser searches, you might get if for a more reasonable amount
And, as always, Lowendmac is still a worthwhile source of contemporary info for older Macs. Whether you can still pick up any of the recommended compatible cards is another matter, of course.
lowendmac.com

JUSTOP Wireless WIFI PCI Card Airport Extreme Compatible Wireless PCI Card For Apple PowerMac G3, G4, G5 (selected models only)
Features: Apple Airport Extreme card alternative, recognize as Airport Extreme 802.11G card under MAC OS Airport software compatible, no driver install required under MAC OS X Compatible with the both 802.11g / b wireless standard High-Speed Wireless data rate up to 54Mbps 54 Mbps standard ...
www.amazon.co.uk

AIRPORT EXTREME WIRELESS-G PCI CARD FOR APPLE MAC POWERMAC G3 G4 G5 | eBay
FOR APPLE POWERMAC. Fit into the PCI slot in your Powermac. If your Powermac G5 is Dual core or Quadcore model, then it's not compatible with this card! Power Macintosh G3 All-in-one, Blue & White, Desktop, Minitower, Server.
www.ebay.co.uk
I know that Sonnet had an Aria Extreme PCI card (N80211-PCI), which was 802.11n compatible. Not sure if the G3 would be the best match for it as it might have required Leopard although you could hack Tiger to work with it on a G5.

Sonnet Aria extreme nPCI Apple Mac Wlan-Karte - 3 Antennen - n PCI - Wi-Fi 802.1 | eBay
Finden Sie Top-Angebote für Sonnet Aria extreme nPCI Apple Mac Wlan-Karte - 3 Antennen - n PCI - Wi-Fi 802.1 bei eBay. Kostenlose Lieferung für viele Artikel!
www.ebay.de
This one is pricier but if you recognise the card on your browser searches, you might get if for a more reasonable amount

Airport Wireless PCI Card for Apple Power Mac G3 G4 G5 | eBay
Fully Compatible with : Apple Power Macintosh G3 (Blue & White), G4 Power Mac (all models except Cube) or G5 (tower, PCI model only ) computers, witha spare PCI slot, running Mac OS X 10.2.6 with AirPort installer, or Mac OS X 10.3 Panther, 10.4 Tiger & 10.5 Leopard.
www.ebay.com
And, as always, Lowendmac is still a worthwhile source of contemporary info for older Macs. Whether you can still pick up any of the recommended compatible cards is another matter, of course.
802.11g Extreme Wireless for Older Macs | Low End Mac

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