Where can I download an OS 9.1 bootable utility to reformat an old iMac?

IOS 9.1 is not Mac OS X I think you just made a little slip there. The only previous system you can buy would be Snow Leopard

http://www.apple.com/shop/product/MC573Z/A/mac-os-x-106-snow-leopard

But you don't say how old?

That is not iOS 9.1 which only runs on iPhone, iPad or iPod. It is OS X 9.1 which is very old. Your only hope of finding a CD set for that would be Amazon or eBay now, since Apple dropped support for it years ago.

Again, the iOS in your profile confused me. I had a Blueberry and remember OS 9 well. As Kappy says, I don't think you can download but if you no longer have the install disks you can probably purchase them online. No Snow Leopard, the best I was able to upgrade mine was OS X 10.3.

An iMac does not run iOS. It runs OS X instead.

If you provide more details we might be able to help you figure out exactly what version of OS X your Mac is running.

Try Amazon and eBay, but I doubt you will find a copy you can download.

Old iMacs did run MacOS starting with 8.1. The models released before 2003 all ran MacOS. Some also ran OS X.

I was going by the iOS 9.1 in his profile.

He seems confused and I was hoping for a reply with additional information.

I have the complete floppy set and download files for OS 9

I believe Kappy is correct. I doubt you'll find anything downloadable but someone might be selling disks. I wouldn't buy them without some guarantee the disks will be good.

Try this http://macos9lives.com/smforum/index.php?topic=2109.0

Hope I'm not violating the TOU.....

Well shut my mouth, Mike! 😉

I had a Rev. B iMac. Ran it 24/7 for 9 years before it failed. I'd be curious if you get the .iso to work and how.