You'll need USB to MIDI adapter compatible with Mac Os 9 like this:

http://www.ebay.com/itm/M-Audio-Midisport-Uno-1x1-midi-interface-USB-to-...

And you can take a look in:
http://macos9lives.com/smforum/index.php

not all midisport unos or 1x1s' will work with os9:
http://macos9lives.com/smforum/index.php?topic=163.0

more detail on midi providing hardware, research by myself earlier this year:
http://macos9lives.com/smforum/index.php?topic=67.0

see these posts by me!

http://www.m-audio.com/index.php?do=support.faq&ID=dafbe23c0f3791fd6e457...

make sure to review this info .. the m-audio branded midisport unos with clear blue plastic is not what u want.. u want hte lighter pale blue branded solid non see thru plastic branded MIDIMAN units..
these ones will work with os9.. the newer ones may work depending on this "chip location" but there is a risk u will buy and find out its useless under os9 and the driver will not work.

i have a roland um 880 8x8 usb midi interface which has os9 drivers ... one of many of these type of devices... be sure to check my 2ndpost from above.. i have tons of links to usb controllers that have drivers that support os9..

it was also possible to retrofit these imac g3s with a native oldschool 'serial port' for midi
see the info here:
www.geethree.com/support/supportstealth.html
http://www.geethree.com/stealth/index.html
http://www.geethree.com/stealth/install/imacinstall1a.html

this enables connectivity + compatibility to/with all oldworld original mid 90s non usb midi devices

http://www.ebay.com/itm/M-Audio-Quattro-USB-Recording-Interface-24bit-96...

http://tinyurl.com/n5uezy9 <-- image

this would work with your g3 mac (has one pictured on the box for that good reason)
provides audio in/out + midi.. everything u need to get started on an imac g3 + would work in
both osx + os9, on both of your macs! im 90% sure

Professional 4-in/4-out audio interface with MIDI and USB.

Genuine pro-quality sound, up to 24-bit/96kHz, through your computer's standard USB port. Pristine A/D/A converters. Multitrack recording and mixing without PCI card installation, zero-latency direct monitoring for simple recording and dubbing, balanced 1/4" TRS connectors, and selectable in/out levels to accommodate different instruments. 1x1 USB MIDI interface.
Features

Genuine pro-quality sound
Up to 24-bit/96kHz, through standard USB port on Mac or PC
Pristine A/D/A converters
Multitrack recording and mixing without PCI card installation
Zero-latency direct monitoring
Balanced 1/4" TRS connectors
Selectable in/out levels

Audiosport Quattro MIDI/USB Interface Specifications:

System Requirements (Mac):
Native USB compatibility
G3 or G4 running OS 9.1
96kHz Operation 128 MB RAM
48kHz Operation 64 MB RAM
OMS 2.3.8 for MIDI
System Requirements (PC):
Windows 98SE, Windows ME, Windows 2000, Windows XP
UDMA EIDE or SCSI HDD
96kHz Operation Pentium III 500MHz, 128 MB of RAM
48kHz Operation Pentium II 400MHz, 64 MB of RAM
UDMA EIDE or SCSI HDD

additionally this interface can be connected to the m-audio omni io to extend its routing capabiltiies + add hardware inserts + direct outs + other features such as:

Features

Two mic/inst and two line-level balanced/unbalanced analog inputs to the Delta 44 or Delta 66 soundcard
Two mic/inst inputs equipped with XLR phantom power, insert I/O jacks, individual gain controls, gain reduction "pad" switch, and signal/clip indicator LEDs
Low impedance mic inputs provide 66dB of gain and utilize M-Audio's critically acclaimed DMP2 preamp technology switchable to hi-impedance instrument inputs with 46dB of gain
Mic/inst preamp has a frequency response of 10Hz to 200kHz, low THD of .0009%, with a dynamic range of 130dB
Stereo effects return
Mixing section all balanced/unbalanced on TRS jacks
Four Delta outputs can be used simultaneously as independent balanced/unbalanced direct outs
Four stereo aux inputs for MIDI devices
Mix out
Control room outs with independent level control
Two headphone outputs with individual level controls
One mono effect send per channel

more on the omni io:

A full-featured front end for your Delta 44, Delta 66, or Audiosport Quattro interface cards.

Makes any routing path possible, like a split-console mixer. Lets you monitor playback, add effects, and perform mixdowns. Eliminates the need to reconfigure between record and playback. Integrates seamlessly with all of today's music production tools, streamlining your desktop studio. Utilizes M-Audio's critically acclaimed DMP2 preamp technology. 2 switchable mic/inst (66dB to 46dB of gain) and 2 line-level balanced/unbalanced analog ins (for the Delta 44 or Delta 66) accommodate mics, guitars, and keyboards. 2 XLR ins with phantom power. 4 stereo aux inputs for MIDI devices. Individual gain controls, gain reduction "pad" switch, and signal/clip indicator LEDs. 4 Delta outputs can be used simultaneously as independent balanced/unbalanced direct outs. Also has mix out and control room outs with independent level control for monitoring. 2 headphone outputs with individual level controls. One mono effect send per channel. L/R output signal/clip LEDs.

http://www.ebay.com/itm/M-Audio-Quattro-USB-Audio-Interface-and-Omni-I-O...

heres a listing that shows the two units together + more pics

u cant go wrong for the price u could grab these for!

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=291014622599&ssPageNa...

heres another os9 interface for sale, the omnistudio usb!!
has some interesting routing capabilities frm what i can tell!