Is there a power bank available for iMac?

There is no iMac power bank as such. Since iMacs are powered by standard home power outlets your option would be to buy a UPS (uninterruptible power supply) or consumer grade power bank. The size would depend on how long your power would be cut off. If just for an hour or so either a UPS or a power bank would do. If the power will be off for a long time, say a couple of days, you would need a gas powered generator or solar generator to keep your iMac running as well as any appliances in your home you depend on.And then there’s the matter of your internet connection. Your modem/WiFi router would need to remain powered also.

None of these options are particularly cheap. You’l be spending north of $100 at least.

As far as I know an iMac cannot be charged or powered by a 5V power source. An iMac has no internal battery to be charged but has a power supply that gets power from a standard outlet. If you know different please provide documentation.

if it's a model that charge by 5volts then it's just a question of getting a cable with the right connectors then all power banks would be available for imacs, of cause how many minutes the power bank would last would very much depend on the amp hours it could dish out