Fujitsu appears to still make some USB and Firewire MO drives that are backward compatible with old 128MB 3.5" MO disks. Their Hong Kong site lists a few that claim Mac compatibility, and a quick Froogle search turned up apparently the same drives with a different model number for sale in the US (around $200).
They're rather uncommon outside Japan, but a little research and you can probably come up with something that'll work if you're willing to pay for it. Just be sure it reads all 3.5" MO disks, not just newer ones (say, 640MB or 1.3GB), before your buy.