A "retail version" is the boxed, retail or universal installer disk that is designed to install on a variety of Apple computers, whereas a model-specific Restore disk contains a software build that supports the model that it shipped with. While some may have success using a Restore disk, it's not the preferred way to achieve a problem-free installation. You can search the web or eBay for a retail/universal OS X (Panther or Tiger) installer disk, which will be black and have a large silver "X" across the face of the disk.
Before you install OS X on the G3 iMac, you must first determine what firmware version the computer has been programmed with. Depending on whether you have a tray-loading or slot-loading optical drive, the required firmware version differs. On that chart, you can see that a tray-loader requires firmware version 1.2, but a slot-loader requires version 4.1.9. If needed, either of these can be downloaded. With the slot-loading G3 iMacs, you should be running OS 9.1 (which you indicated), in order to run the firmware updater. Additionally, the specific model G3 iMac that you have determines whether you'll be limited to installing OS 10.3.x (Panther) or OS 10.4.x (Tiger). The hardware requirement for installing Tiger on a slot-loading optical drive G3 iMac would be FireWire ports. Be aware that the retail/universal Tiger installer disk is a DVD, so even if you have an iMac with FireWire ports, an internal DVD-ROM drive or an external FireWire-connected DVD drive would be needed. Some of those iMacs that support Tiger had a CD-RW drive. You'll also need adequate system memory installed, as well as adequate hard drive capacity.