Apple once made the best keyboard in the world and it works on PowerMacs with ADB ports like yours--the M3501 Apple Extended keyboard, or "Nimitz," nicknamed for the fact that its size remineded someone of the flight deck if the US aircraft carrier of the same name. If you plan to use the computer for serious writing tasks, the Nimitz is true writers' dream.
Check with schools. Many have a stash of old Mac bits and they may be willing to give one away.
Another possibility is to get involved in a local Mac Users Group. Tht ould eliminate the shipping expense.
http://appleusergroupresources.com/find-a-group/
Another option is to find an older USB PCI adaptor card and go with a USB board and mouse. When I added USB to my Beige G3 Minitower, it extended its usefulness by several years. Just avoid such cards made by Belkin, and any that don't have 'NEC" stamped prominently on the largest chip on the adaptor card. Orange Micro, Sonnet, and Macally sold USB PCI adaptor cards with the NEC chipset.
This is a "known-good" version:
http://www.ebay.com/itm/MacAlly-UH2-225-5-Port-USB-2-PCI-Card-Mac-/361164628739? pt=US_Internal_Port_Expansion_Cards&hash=item541716e303